Relationships, particularly romantic ones, tend to thrive when the people involved are similar. People are simply more likely to get along if they share common interests, values, beliefs, and goals. Thus people with similar personalities will often flow together easily.
The dynamic of this pair differs widely from Opposites Attract pair. Their personalities can often be interchangeable and both of them can have a same role if they are a part of a group. However, their common obstacle is having a harder time to compensate for each other's weaknesses.
Be warned, writers: while often Truth in Television, this trope can sometimes be seen to create a poorly written Love Interest who just happens to be a Gender Flip of the main character.
When the commonalities are much fewer but still create a bond, see Commonality Connection. Contrast Too Much Alike.
Not to be confused with the Sitcom of that name.
Popular Birds Of A Feather dynamic tropes:
Lenalee and Allen from D.Gray-Man are pretty similar in personality (especially in the manga). They're both pretty serious (although Lenalee is a bit more so), both have moments of Get a Hold of Yourself, Man! (with one slapping some sense into the other, and the other apologizing and thanking them), and they both have very similar goals and views. Also, love relationship-wise, they're both very not aggressive, and tend to be extremely shy. If they were to get together, it would be pretty hard to say who would wear the pants in the relationship, honestly.
Train and Saya from Black Cat became very similar. Initially, Train, although having a nice nature at heart, repressed much of his feelings and wasn't very expressive other than being angry. However, after spending some time with Saya, he quickly became Genki like her, even choosing to go after the same job as her and apply the same principles and morals she had. Near the end, Train becomes so much like Saya to the point where Creed actually sees a ghostly image of Saya juxtaposed on top of Train. Although their relationship might have possibly evolved into a romantic one, it didn't.
Tsuna and Kyoko from Katekyo Hitman Reborn! aren't really very different in personality. They're both determined, both naive and good natured, and both very oblivious to other character's feelings for them (In Kyoko's case, Tsuna's feelings for her; In Tsuna's case, Haru and Gokudera's feelings for him). It doesn't help that due to Art Evolution, Tsuna and Kyoko are now practically twins.
Tsuna's new best friend Enma is basically himself. That makes them not only the "Loser Duo", but also the "Amazing Duo".
In Code Geass, Lelouch and C.C. resemble each other in many ways. They both have Geass powers, they are both angsty, both have problems with their admirers / love interests (Mao and Shirley, respectively, and later Rolo), and both tend to have rather calm demeanors. One of the traits that is the most similar between them is that they are both very lonely individuals. Lelouch even comes out and says to her, "You're not alone. We are accomplices. If you are a witch, then I am a warlock!"
Suzaku and Euphie. Their goals are the same, they have a very similar sense of humor, and they get serious about the same things. Not to mention a moment in the anime, where they're shown to say the same thing at the same time, only to both burst out chuckling.
In the manhwaThreads Of Time, Atan Hadas falls in love with protagonist Moon-Bin because (due to being from the future) he is open-minded about women's rights, kind hearted, and dislikes unnecessary violence. Because she is an outgoing Tsundere who wishes people would see women as equals, she finds him refreshing and wonderful, calling him her "precious confidant," and loves talking with him because they have similar views. To further this trope, she hatesSali Taiyi because of his chauvinistic views and horribly violent ways.
Momoka and Tamama are described as being this in Keroro Gunsou. Both of them seek love from someone else but fail, and both have split personalities.
Does it still count if they're not in love with each other?
In Full Metal Panic!, this is one of the reasons Gauron implies to be why he's Sousuke's Stalker with a Crush. In many ways, they are very similar. They're both war veterans that went through very violent, bloody battles and killed many people. They're both experts at piloting AS. They're later shown to both be working for organizations that hire mercenaries (though their organizations are always opposite and always clash with each other, making them destined enemies). Of course, the main difference is that Gauron is Ax-Crazy and Sousuke doesn't care for murdering people.
One of the more recent arcs of Naruto has the title character develop a Intergenerational Friendship with Killer Bee who, with all intents and purposes, is an adult version of Naruto.
This appears to be the main appeal of the Greece/Japan pairing in the Axis Powers Hetalia fandom (well, aside from the "private lesson"). They both have quiet, subdued personalities and share a love of cats, and they're about the only teased-at couple that doesn't have a Tsundere or clear-cut Opposites Attract dynamic.
What makes this interesting is that the two are actually opposites in many ways (they do come from separate continents and cultures, after all) but are still similar enough at their core (it's amazing how well a lazy, spacey Greek who Really Gets Around and a hardworking, focused Asian who Hates Being Touched will get along when said Greek is a Nice Guy and said Asian a Yamato Nadeshiko deep down) that they don't butt heads over their differences like most other couples/pairings from the series do, but instead accept or even appreciate them.
Non-romantic example from Fullmetal Alchemist- Sig Curtis and Major Armstrong hit it off immediately, both being Gentle Giants who love flexing their considerable muscles; Armstrong's sister, Olivia also immediately gets along well with Sig's wife, Izumi, as both of them are Ladies of War with a propensity to demonstrate Good Is Not Nice.
Ranma and Akane from Ranma ½ could be described as this. They're both stubborn, headstrong, competitive, short-tempered, forgiving, martial artists, etc. Whether those similar traits are the very things that attracted them to each other in the first place is up for debate.
Mobile Suit Gundam Unicorn: Banagher and Audrey. One of the reasons why they bond quickly is the revelation that both spent the majority of their childhoods on the move. Audrey/Mineva and Riddhe develop this too when they both discover that they're trying to break away from the continuity of their respective families' past professions. Banagher also develops this later with Marida when they find out that they're both Cyber-Newtypes.
Except that Banagher ISN'T a Cyber-Newtype at all, he's a perfectly natural one. All Marida said was that they might not be that different, after all Banagher saw inside of Marida and saw that her life was full of sadness (She survived the first Neo Zeon war only to be prostituted and subjected to constant abuse and abortions, until Suberoa saved her). It's more than likely that Marida saw similar trauma in Banagher's life (Thus far the anime suggested his parents doctored his memories, he was taken away from his father when he was young and then his mother died and left him alone). Nothing in the anime has suggested that Banagher is a cyber-newtype and in fact the novels explicitly state that Cyber-Newtypes (like Frontal) cannot activate the La+ system or open Laplace's box.
Both Konata and Kagami from Lucky Star have remarked about the similarities between the members of their little group of friends. Kagami even notices that her friends from her actual class (Ayano and Misao) are similar in some ways to her sister Tsukasa and their friend Konata. And of course, the otaku set, Konata, Patty, and Hiyori, seem to hang out a bit.
Hei and Kirihara of Darker than Black are both Big Eaterstoics with a good heart who care about their teammates. Kirihara and November 11 both share a loyalty to teammates and a hatred of smoking, although November 11 is more... vehement about the latter.
Near the end of Neon Genesis Evangelion, a fair bit of time is devoted to showing how both Shinji and Asuka have the same sense of abandonment and need to be acknowledged, but react in different ways.
In ...Virgin Love, the two leads' Ethical SlutWork Hard, Play Hard personalities and their ablility to relate over work is what draws them together. Daigo describes it as finally having found someone who can meet him on the same level. They also both share the weakness of Pride.
Although they used their similar backgrounds to create different character traits, Hinagiku and Hayate of Hayate the Combat Butler have the similar pasts to make themselves candidates for this trope.
Recently, Luca has been added to these two, with a similar past, and adds more than a little of Hayate's looks to seal the deal.
Kotetsu Kaburagi and Tomoe Amamiya from Tiger & Bunny became friends upon discovering they were both superhero geeks. Worked so well that they became Highschool Sweethearts, then got Happily Married. Too bad poor Tomoe died of illness some years later.
Mahou Sensei Negima!: (Possibly) one-sided version with Negi and Nodoka: both are quiet, unassuming, intelligent, easily flustered Cute Bookworms. They are also two of the most dangerous members of Ala Alba, with unexpected but increasingly obvious Badass sides.
In Saint Beast, this is almost literal with Rey and Luca who are brought together by their ability to fly.
In Claudine, the main character and the local Hot Librarian Cecilia were united by their love of reading and poetry.
Naruko/Anaru and Atsumu/Yukiatsu in Ano Hana. They both blame themselves for Menma's death, have inferiority issues (Anaru, in regards to Menma; Yukiatsu, in regards to Jintan), and are "kindred spirits" in unrequited love. This is why Yukiatsu saw Anaru as a possible back-up girlfriend, too.
In Popcorn Avatar, Ren/Agni states the real reason why Lisa/Vayu and Amano/Indra tolerate each other is because they're both violent and have rotten personalities to begin with.
Judai/Jaden and Johan/Jesse from Yu-Gi-Oh! GX are so similar, they become quickly best friends. Judai basically even abandons his other friends only to save Johan.
Conrad/Yuuri is the pairing if you like this in Kyo Kara Maoh. Among the various guys who have Ship Tease with Yuuri, Conrad is the one with the most similar personality, and he tries to take an active interest in the things Yuuri enjoys, like baseball.
In Magical Girl Lyrical Striker S, Griffith and Lucino are in good terms with each other, as both have a calm and serious personality, and purple hair. Their relationship is very subtle because they are just minor characters, but it's strongly implied that they have married between StrikerS and FORCE, as Lucino has taken Griffith's surname, Lowran.
This was a recurring theme in the first issue of Heroes, Anonymous, where former sidekick Attaboy's struggles to make it as a solo hero are helped when he discovers a kindred spirit, Lynn, who eventually becomes his girlfriend as they bond over an old sitcom from the 80s, "Birds of a Feather." Except Lynn is easily the quirkiest living being on the planet (she's absolutely obsessed with Birds of a Feather and the only music she listens to is whale songs), while Attaboy's attitude is something along the lines of I Coulda Been a Contender.
Scott Pilgrim and Ramona Flowers have many of the same flaws, such as not being totally decent to their past loves. It comes to a head in volume 6, when Ramona tells Scott what she did during their separation — exactly what Scott did: dicking around, watching reruns, and sleeping. Scott's friends declare them the perfect couple, although it has less to do with them being perfect for each other and is more like they deserve each other.
Scrooge McDuck's sister Hortense was well-known for her short-temper and outburts of anger. So is Quackmore Duck. When they first meet, they get in horrible argument...and immiediately fall in love. Needless to say, this quality gets passed down to their son.
A Growing Affection does this for Naruto and Hinata, giving them a few interests in common outside of ninja life (swing dancing and fantasy novels among them), and a few close but not quite interests (they like romantic comedies, but for different reasons). Sakura also claims (in a fit of jealousy and sadness) that the only reason they are together is because they are both losers as ninja.
The driving force behind Lindsay and Justin' relationship is that both were astounded at finding someone else as hot/gorgeous as themselves.
After the Awake-a-thon, it comes to light that the obese and outgoing Sadie has been making eyes at the obese and outgoing Owen.
In Amélie, both Amelie and Nino are quiet little oddballs who are very kind but aren't all that good at making friends.
James Bond and many Bond Girls are like this. Both being suave, sexy, seductive, and many times, they share the same occupation and goal. This is the same for the novels to a degree. One reason Bond falls for Vesper Lynd in Casino Royale is that he finds her easy to talk to and open up to, and they find they have a lot to talk about.
John le Carré's conception of Bond adds another element to this. Le Carre disliked Bond and said he's perfect traitor material (being that he actually worked in MI6 he'd probably know), Vesper is in fact a traitor.
Both couples in White Christmas, the mature Bob and Betty in contrast to the outgoing Phil and Judy.
In Mulan II, Yao, Ling, and Chien Po are told by the Matchmaker that they will never find girlfriends because of their "lack of personalities." Not only do they find girlfriends with personalities like their own—Yao's is feisty and rebellious, Ling's has a corny and immature sense of humor (though she tries to hide it), and Chien Po's is a sweet Big Eater—but they are princesses.
In Sense and Sensibility, Marianne acknowledges Edward is suited for Elinor because of this trope, although having a reserved lover like Edward would disappoint Marianne herself, who longs for someone who shares her taste in music and art; Colonel Brandon's Establishing Character Moment happens to include a Meaningful Echo of her words.
A great part of the point of Pride and Prejudice is to show that even if Lizzie and Darcy are both judgemental snobs with a certain disregard for politeness, they can show a great deal of kindness and loyalty and are much more clever and single-minded than any other character, thus being perfectly suited for each other once they smoothe up.
Bookworms Catherine Morland and Henry Tilney of Northanger Abbey.
This is a significant theme in Gone with the Wind. Scarlett spends the majority of the novel lusting after Ashley, who is different from her in every way that counts. Even though a bunch of characters, most often her father, tell her that like needs to marry like it takes her until the end of the novel to figure out she would be happiest with roguish Rhett Butler. Of course, by the time she clues in, he's sick of her.
Good Omens: Anathema Device and Newton Pulsifer have both given their vehicles names (hippy Anathema's is a bike; loser Newton's is the worst car ever). They're literally destined to hook up.
Harry Potter and Ginny Weasley are this way, both being fierce, Quidditch-loving Leos, with a darker sense of humor. This is in contrast to Ron/Hermione, who are Opposites Attract.
Neville and Hannah Abbott. Though their relationship was only brought up in interviews, Hannah has been stated multiple times to share Neville's interest in Herbology, and was a staunch member of Dumbledore's Army from the very beginning, meaning they have undoubtedly spent a great deal of time together off-screen and have quite a bit in common. They also both had some self-esteem issues growing up (see Hannah's panic attack in the run-up to O.W.L.s).
In The Hellfire Club by Peter Straub, this is the reason why villainous Serial Killer Dick Dart claims he loves and adores Nora. Because of a misunderstanding on his part where he thought she had kidnapped a woman and tortured her horribly for several days, he comes to believe that they are alike. He even comes out and says that he's so happy he found a female version of himself. In wit and many other ways, they are shown to be a worthy match... the only thing is that Nora is not fond of killing people in bloody ways.
In Andersen's The Shepherdess and the Chimney-sweep, the titular Star-Crossed Lovers are like this:
They had both been put on the table where they stood and, having been placed there, they had become engaged because they suited each other exactly. Both were young, both were made of the same porcelain, and neither could stand a shock.
The In Death series: Winnie and Moriarity from Indulgence in Death are very much this. In fact, one woman says that those two are not two sides of the same coin, but they are the same side!
Audrey, Wait!: Audrey and James are huge music enthusiasts, and this is their go-to conversation topic.
Caine and Diana, from Michael Grant's GONE series, are more or less male/female counterparts of each other. Both lying, manipulative, selfish, apathectic sociopaths, even Diana admits in LIES that the only reason they loved each other is because no one else would be insane enough to love them because they're both sick and evil;
"as sad as it was to realise, he was all she had. The two of them. Sick puppies. Twisted, arrogant, cruel and cold, both of them. How could she love someone like that? How could he? Process of elimination? Neither of them could find anyone else? Even the nastiest, ugliest species found mates."
Worth noting that they even ''look' alike. They're both described as skinny, caucasian brunettes with dark brown eyes and are apparently both very pyshically attractive (even the heroes admit this) until starvation sets in.
Live Action Television
In the first season of How I Met Your Mother, Barney tries to seduce Robin, saying that "we both think commitment's a drag, we want something fun and casual, and we clearly get along". They didn't get together then, but jump to season four, where they're firmly in Will They or Won't They? Land. By season eight, they are engaged with season 9 about their wedding.
More so in season 9 but we see how alike Ted and the titular Mother are. Both are the Nice Guy and Nice Gal, have driving gloves, knows facts that only each other would find interesting/funny and loves going roadside to see obscure things. And the Mother's First Love Max died in 2005 (the same time Ted met Robin), leaving her unable to move on in any relationship, similar to what had happened with Ted and Robin.
Though the downside of this trope is also shown with Quinn and Barney. While they got along well enough initially as they truly got each other and loved manipulating others, they eventually realized they couldn't trust each other.
Chuck and Blair on Gossip Girl are basically a male and female version of the same character.
In a completely different way, Blair and Dan.
Topher and Bennett of Dollhouse serve pretty much the exact same function in their respective houses.
It can be that AND Birds of a Feather. They recognized each other's species and presumably began to date as a result of having that much in common.
Blutbad too. Granted with the wolf like nature of their species it is safe to say it is similar to how pack mentality works. It's natural for them to gravitate toward one another as in the case of Monroe, Angelina, Hap and Rolf.
Ironically despite being from different "species" of wesen, Rosalee and Monroe seem to be heading in this direction themselves. Both appear to have calm collected dispositions. Both are rehabilitated and are attempting to live normal lives. It would also appear that like Monroe, Rosalee has no qualms about doing mundane tasks like maintaining a herbal shop.
Seth and Anna on The O.C. were this - it ended up being the cause for their break-up.
Wendy and Tyler in The Middleman are both geeky artists with a high potential for heroics (even if Wendy was the one eventually chosen for the job).
All of the good, long lasting couples on The Office. These include Jim and Pam, Michael and Holly, Andy and Erin, Dwight and Angela, along with secondary couples like Bob Vance and Phyllis.
Friends: Chandler and Monica manage to be this andOpposites Attract. He's relaxed, calm and committment phobic she's energetic, uptight and looking for a serious relationship. However they both suffered difficult childhoods and are pretty neurotic. The key is that while their reactions are different, their issues are similar and so they can understand each other. (Which is very much Truth in Television). This conversation sums up their shared issues well:
Chandler: Is that why you became a chef? So that people would like you?
Monica: Oh, you really want to talk about getting people to like you huh, funny man?
Interestingly Monica's Second Love Pete was definitely Birds of a Feather. Like Monica he was a Competition Freak and The Determinator, which actually caused their break up, hinting Monica couldn't be with someone as stubborn and driven as she was. Her next and final boyfriend was Chandler who repeatedly demonstrated he was capable of stabilizing her.
Ross and Julie, Ross and Charlie and even Ross and Emily are also this. (Contrasting with Rachel).
Carly and Freddie on iCarly, especially when compared to the other member of their Power Trio, Sam. Most notable in iSell Penny Tee's with the contrasting styles in running their t-shirt factories. Sam runs her 'factory' like a child labour sweatshop, while Carly and Freddie run their way by being nice and fun.
Jack O'Neill did poke fun of it in an episode of Stargate SG-1. "It has to do with flocking... and togetherness."
As of The Doctor's Wife we can add the TARDIS to the list.
Sybil and Branson on Downton Abbey They are initially drawn to each other due to their shared interest in politics and women's rights. Later episodes reveal their rebellious natures and disregard for societies class divisions.
John Sheridan and Ambassador Delenn from Babylon 5, whose interest in each other quickly begins to heat up when they discover that they share not only a gift for military and political strategy, but an innate curiosity about other races.
After Faith returns in Buffy the Vampire Slayer she sneaks off for a smoke, where she is found by Spike. Sharing the cigarette they discuss the potential slayers that are being trained up, going evil and Buffy. Their shared experiences (as well as a rather saucy comment Faith made while pretending to be Buffy that Spike had not forgotten) leads to a mutual understanding, respect and likeness for each other.
Call the Midwife has Dr Turner and nurse Sister Bernadette, both kind, dedicated healers devoted to a common cause and very much cut from the same cloth, dubbed "Turnadette" by fandom. The fact that Sister Bernadette is also a devoted nun, however, means that she spends the second series torn between her love and her vows until, after a bout of TB and a lot of soul-searching, she leaves the convent and he asks her to marry him.
Shepard: This won't work, Miranda. We're too much alike.
Fire Emblem often gives support conversations and/or common endings at A support level to people who are very similar in personalities. I.e., we have the cavalier Kent and the pegasus knight Fiora: both are serious, protective, kind characters who bond over their similarities:
Fiora: I am Fiora, the pegasus knight. You have been looking after my sister. Allow me to thank you.
Kent: Oh, yes... When we were in Caelin, Florina spoke of you often...
Frederick and Cherche from Fire Emblem Awakening. They're both completely loyal to their lords (Chrom and Virion, respectively), workaholics that do all the menial chores said lords can't be bothered with without a single complaint, they're both quite well-spoken and abide by strict knightly virtues.
Rather hilariously subverted with Henry and Tharja. Both are very strange-minded Dark Mages who love fighting out there, but he's a Perpetual Smiler and she's a Perpetual Frowner — meaning, they're both this andOpposites Attract. And yet Tharja says they swear by this trope if they hook up, since they're not the most normal people anyway.
Bioshock Infinite gives us Robert and Rosalind Lutece. Both are absolutelynuts. and there is a subtle attraction between them, particularly if you read Rosalind's voice logs. As they are alternate universe representations of the same person, it borders on Selfcest.
Homestuck: Rose and Kanaya are both very intelligent, classy, snarky women who have a tendency to cover up their own emotions but are perfectly capable of taking care of themselves. And indeed, later on they do get together.
Discussed and Subverted in Saturday Morning Breakfast Cereal's "Uncomfortable Truthasaurus!" by Zach Weiner. Misconception: Opposites attract or like attracts like. Somehow you get both of these. The truth is that 95 % of people want someone hot. 100 % of people just want anyone with a higher social status.
Chara and Enel from The Silver Eye, with both of them being quite immature and light-hearted.
Tris and Chaos (two wacky outlaws), Beth and Corri (two extremely good Jedi), and Moira and Darth Apparatus (two insane Sith) in The Gungan Council.
Jack and Maddie in Danny Phantom. True, one is closer to Earth then the other (three guesses on who the obvious culprit is), but they both share a mad obsessive love for hunting/researching the paranormal field of ghosts.
In Happy Tree Friends, Cuddles and Giggles both love the color pink, and both are really nice characters with hidden depths (Cuddles is rather rebellious, and Giggles gets pissed at polluters.) Plus, even for Happy Tree Friends, they die a lot.
On the Total Drama series, Gwen and Duncan become close after they discover that they share several interests in common. While they insist that they're Just Friends, their respected partners get incredible hostile over it, resulting in two break-ups and one hook-up between the Birds Of A Feather partners.
This seems to be the case with Sierra and Izzy as well.
Zuko and Katara are a non-romantic example of this. Both are passionate, determined individuals willing to do whatever it takes to accomplish their goals, whether it be capturing the Avatar or helping him. Both are also shown to be pretty evenly matched in terms of bending abilities, and the few times they are actually on the same side/not angry at each other they work very well together and get along extremely well, showcased best in the finale.
While they never got together, and firmly remain as Just Friends, this is the most likely reason for why Rogue was attracted to Scott in X-Men: Evolution. Both have dangerous, uncontrollable powers, both lived sheltered lives, and have an older sibling like dynamic with younger students.
Finn and Marceline from Adventure Time are a non-romantic example; who both enjoy adventure and edgy lives.
Goof Troop and its movies used this on several occasions. The episode "Puppy Love" involves Shrinking VioletNice Guy PJ getting into a Twice Shy situation with Shrinking VioletNice Girl Rose Deckenbloom (and if that weren't enough, both of them know how it feels to be belittled). The first movie involved Adorkable Max falling for Adorkable Roxanne and vice versa, and they are both socially awkward in exactly the same way. The second movie actually has two of these: PJ and Beret Girl, the cynical-but-sympathetic beatniks, and Goofy and Sylvia, the Disco Dan duo—and in both of PJ's cases, they shared a common taste in poetry too.
Occasionally offered as one reason behind Unknowing Incest situations. This reasoning is referenced in "Weird Al" Yankovic's "A Complicated Song": "I was / Feelin' pretty down / 'Til my / Girlfriend came around / We're just / So alike in every way, I gotta say..."
There may be some real-life basis to this as it's better in the long term to have someone who shares similar values to you when you get to things like kids, money, religon ect. People also tend to date people who live close to them.