Maybe it's that they're so cute when they're helpless. Maybe it's that we really want to feel loved and protected when we're vulnerable. First aid is intimate. You have to lean in to pat a cut eyebrow with dettol-soaked cotton wool gently. Narrowly escaping a life-threatening situation gives the scene extra zing.
Whatever the case, patients can and do fall in love with those who give them medical care, and vice versa.
When the care-giver is a professional, such as a nurse or doctor, it would be considered extremely unprofessional for a doctor or nurse to act on those feelings. Doctors are advised against getting infatuated with their patients or to clearly rebuff the patients' advances, and having sex (or any sort of overt romantic liaison for that matter) with a current patient or any of their family member is considered a punishable breach of medical ethics (there is a line in the Hippocratic Oath specifically forbidding that) to prevent the doctor from abusing his or her position of authority to coerce sex from patients. But in fiction, this will usually be portrayed as sweet and romantic.
When the care is given by an amateur, there is an added element of romance because the medical treatment is entirely voluntary. This isn't their job, it's a genuine expression of caring for the well-being of another person. Typically, the "civilian" version comes in two flavors:
Previously aloof tough character gets injured, lets the love interest treat their injuries, and reveals a newfound trust and intimacy. This might or might not involve the tough person making that hissing sound when the love interest touches their injured face, but not quite pulling away...
The love interest gets injured, and the tough character treats them and suddenly reveals a tender side.
InuYasha fell for Kikyou after she once got him to let her nurse his wounds. The series also includes a darker take on the trope: Kikyou nursed the bandit Onigumo after he was burned and severely injured to the point that he could no longer move. Onigumo developed a serious case of If I Can't Have You lust for her which led him to trade himself to a pack of demons in order to obtain a demon-hybrid body capable of movement, becoming Naraku in the process.
There was kind of a plot point in Kimikiss Pure Rouge about this, there was tons of UST before that, but when the male lead got sick things started progressing a bit more.
This is how Spike and Julia fell in love on Cowboy Bebop. Subverting this, after Spike gets nursed by Faye, the only thing that happens is Spike gets thumped for complaining about Faye's off-key singing.
Arguably not quite a subversion since Faye does have feelings for Spike, it's just that they're complicated characters and their love-hate relationship never gets a chance to go beyond being mutual attraction and (an almost brotherly-sisterly) annoyance.
In Weiss Kreuz Gluhen, an amnesiac Youji's ultimate fate is to marry his nurse and take her last name.
In Rurouni Kenshin, Shougo Muto aka Amakusa rescued a Japanese boy from some bullies who hated him for being a Christian. The boy joined their Christian community and took up the name Shouzo... and pledged himself completely to Shougo's sister Sayo Magdaria, who nursed him back to health.
Subverted in Shamanic Princess with Tiara tending to Graham's wounds. He goes in for the kiss, but is ultimately denied.
Implied in Naruto with Yahiko and Konan, albeit in a flashback, with Konan tending to Yahiko's injuries while Nagato waits outside, and it is indicated that Konan was in love with Yahiko.
Averted when Sakura (politely) turns down a love confession from one of her patients during the Fourth Ninja World War.
In Dragon Ball Z, despite majority of their Relationship Upgrade happens during a Time Skip, this is implied to be the reasoning behind Vegeta and Bulma's eventual hook up. She nurses him back to health after he over exerts himself during training.
In Haou Airen, an Ordinary Highschool Student named Kurumi Akino finds a handsome guy seriously injured in her way home. She tends to his wounds out of kindness... and few afterwards she finds herself kidnapped and shuttled to Hong Kong, since her "patient" Hakuron just happened to be a high-class member of The Triads and the Tongs and now he wants to "repay" her via making her his concubine.
Ranma ½: Almost every time Ranma's been hurt after a fight, Akane's the one to bandage him up and stay by his side. One of their first romantic moments together was when she tended to his wounds in the ice skating competition arc.
A cruel exploited trope happens in D.Gray-Man. It is first invoked by a doctor in a local hospital where he encouraged his nurses to romance with the ill patients they are attending. Meanwhile, he secretly poisons the patient through their so-called medicine. When it's too late, the doctor prepares arrangements with the heartbroken nurses to revive the patients. That's where the Akuma spawns are as well as the hospital's fundings came from.
Subverted in Kare Kano. Tsubasa's Hot Dad married a cute nurse... but she actually took care of Tsubasa in the hospital, not of him.
In Les Maitres de l'Orge, a French-Belgian graphic novel series, one of the main characters is wounded during World War I and ends up impregnating and then marrying his nurse.
Occurs in the Garth Ennis war comic Battlefields: Dear Billy, though Carrie and Billy don't actually date until after he's left the hospital.
After Havok returns from the Mutant X universe, in nowhere near mint condition, this occurs between him and his nurse.
It didn't help that he was in a coma while being tended to by his nurse.
It certainly didn't help that Nurse Annie's mutant son was psychically "encouraging" them to hook up.
This happened to Batman and Wonder Woman in JLA, when after some time-traveling caused him to get sick and she took care of him.
In The Golden Age of Comic Books, Princess Diana first became infatuated with the injured, helpless pilot Steve Trevor as she nursed him back to health after he crash-landed on her home, Paradise Island. This served as the catalyst for her transformation to Wonder Woman.
In Sonic the HedgehogPrison Island Break it is very obvious that this has happened with Shadow the Hedgehog, who is obsessed with Amy. It's not very romantic, and in fact is even a little creepy since Shadow is a serial rapist-murderer, and voices his desire to rape her specifically right to her face. She tries to rebuff him, or ignore his advances, but it seems she's quite fond of him too.
Comes up in the Pony POV Series in Shining Armor's side story. Private Garnet Chambers treats Private Running Gag for a concussion, during which he develops a crush on her; a later chapter shows that she apparently now returns his feelings.
Rainbow Dash tries to deliberately set up this scenario in How Every Shipfic Would Actually Happen, by deliberately breaking her wing with an anvil and begging Twilight to heal it with her love. Twilight is having none of it and just drags her off to the hospital instead.
Back to the Future is where most of us heard about it the first time: Marty is hit by a car and nursed by the teenage version of his mother. This results in her developing a crush on him. In the original version of the timeline, that was how Marty's mother fell in love with his father. Awkward.
Probably the world's only example of SlapSlapGauzeGauzeSlapSlapGauzeGauzeSlapSlapKiss: Jen and Lo of Crouching Tiger, Hidden Dragon. They fight, Lo treats Jen for dehydration, Jen knocks him on the head with a rock, Lo treats her for dehydration again, and while he is treating her bloodied feet, she tries to regular assault him before she sexually assaults him (with his cooperation). Bonus points because Jen even makes the hissing noise as he picks a splinter out of her foot, but does not quite pull away.
This is how the Mariachi gets introduced to Carolina in Desperado after the Tarasco Bar shootout.
The English Patient does feature a nurse whose patients sort of have a crush on her, but it's fairly anecdotal. The titular English patient doesn't fall in love with her—his one true love died years earlier.
The German movie The Princess and the Warrior has the female protagonist, a nurse at a psychiatric hospital, masturbating one of her patients during the night shift. The scene is rather squick-inducing when you discover the patient is her father.
Sarah Connor and Kyle Reese in The Terminator. Reese gets shot in the preceeding scene and Sarah notices it after what must have been an hour or so seeing as how they drove until they ran out of gas. Cue Major Injury Underreaction from Reese. Sarah flips out of course and proceeds to bandage up his arm, apparently oblivious to the obvious Male Gaze she's getting. It's a pretty important scene for their relationship, as Sarah finds out Reese's first name. Bonus points for hissing sound, btw.
Tom Jones, which gets extra points because Tom got injured while saving the girl from a mad horse.
In Quo Vadis, Lygia helps nurse Vinicius back to health after he is almost killed by Lygia's bodyguard, Ursus, during an attempt to kidnap her. This eventually leads to Vinicius' conversion to Christianity, which makes a relationship with Lygia possible.
In Wraith Squadron the Wraiths speculate when one of their own, Falynn, verbally defends another behind his back. One says she's sweet on him. Another says she is merely responding to his pain, like to a hurt animal, and she wants to nurse him back to health. They lay down bets. A third says she thinks it's a little from Column A, a little from Column B, and some women see a man who is a mess, feel the urge to repair his problems, and then fall in love with him while they're working on him.
"Emotional distress as an attractant. Say, Tyria, I have a sharp pain in my childhood memories."
"What a terrible line. I wish I'd thought of it."
Lensman ("Galactic Patrol"). When Lensman Kimball Kinnison is severely wounded, his admiral orders that the most attractive nurses in the Patrol attend to him, figuring he'll inevitably fall in love with one, so she might as well be a looker. The doctor criticises this popular belief, pointing out that while patients fall for their nurses, the reverse seldom happens because the nurses aren't seeing the patient at their best. Nevertheless the stunning Clarissa MacDougall attends to the handsome hero, and sure enough the two proceed to annoy the hell out of each other.
In this case, it later turns out to be a bit more complex, since Kimball and Clarissa's marriage has been planned as part of the Arisian plan to produce humans psychically powerful enough to become Third Stage Lensmen. The number of people - or rather, entities - who had an interest in getting them together is rather high...
It also goes deeper than the skin. Surgeon-Marshal Lacey isn't picking his nurses on the basis of their looks, but on their skeletal structure - he has a theory that the skeleton tells all, and that a more "perfect" skeleton indicates greater human potential. In a way it's a twist on phrenology, but it's justified in-universe (unknown to him) because of the fact that the Arisian breeding program has created the acme of both physical AND psionic/mental perfection in its penultimates, so of course the best skeletons will "match".
Kara and Belknap in the Ravenor books. They met when the titular Inquisitor had to bring a medic into an undercover operation to patch Kara up after a rather nasty gunshot wound.
In Robertson Davies' Fifth Business, the hero Dunstan becomes involved with one of the nurses attending him while he's in an army hospital. Disturbingly, his stated reason for not wanting to marry her is that he doesn't want another mother figure in his life.
Lessa of Anne McCaffrey's Dragonflight bandages up the wound F'lar received while dueling in her defense. Despite the fact that she does not like or trust him, she can't help but notice his rippling muscles, etc. Sexual tension ensues.
Jaxom fell in love with and later married the nurse who treated him after he fell ill in The White Dragon.
Stanley's father in Holes tells him this how his great-grandfather, Stanley I, met his wife, delusional and thinking she was an angel after he was rescued from the desert he was abandoned in.
In A Song of Ice and Fire, Robb Stark takes a wound in battle and is nursed by a girl named Jeyne Westerling, who he then marries, despite his vows to marry a daughter of house Frey. Doesn't end well for him.
In The Hunger Games, Katniss spends a lot of time healing Peeta after he's injured. Later, he returns the favor.
S. Y. Agnonís short story The Doctor and his Divorcee subverts this: all the patients are very, very fond of the beautiful nurse, but itís the doctor who marries her.
P. G. Wodehouse was fond of this trope and played it straight on several occasions. In at least one story, the Genre SavvyJeeves take advantage of it to set up a couple.
In A Brother's Price, Princess Odelia is injured by thugs and carried to safety and nursed by Jerin Whistler, who she notes is very beautiful. She decides to feign being more hurt than she is - though she is hurt - in the hopes of seeing him some more and maybe getting a chance to steal a kiss. It doesn't work, but hey, First Girl Wins, more or less.
In the autobiographical Going Solo, Roald Dahl recounts falling in love with a nurse who assisted him through a period of blindness after a plane crash in the North African desert during World War II. His infatuation ended once the bandages came off and he found that she was not quite as beautiful as he had imagined her to be.
In the Miss Marple novel Pocket Full of Rye, Percival Fortescue met his wife Jennifer when she nursed him through pneumonia.
A brief version in Atonement. While working as a nurse, Briony has to tend to a French soldier near death. He's delirious and thinks she's an English girl he met in his childhood. Briony plays along with this. Just before he dies, he asks if she loves him and she says yes. The book and the film make it clear that she did, if only for a few moments.
The Earth's Children series by Jean M. Auel provides two examples in Valley of Horses. Both brothers, Thonolan and Jondalar, endure catastrophic injuries and end up married to the women who nursed them back to health.
Live Action TV
House of Harmony: In 1920s Singapore, a Chinese girl who picked up medical skills from her father nurses back to health an American businessman who had been severely wounded by muggers. He falls in love with her, and before long she reciprocates the feeling.
I Carly: Carly towards Freddie in iSaved Your Life. Notable as he become injured by a truck performing the titular saving of her life.
Also done in an earlier episode with Mrs Benson (Freddie's mom) and Lewbert.
Jack from LOST met his ex Sarah when he performed Opsurgery on her to restore her shattered spine. He promised she'd dance at her wedding. She did.
Arrested Development had a nurse that fell for coma patients. It would turn out that they were faking it, and when they admitted they were conscious and reciprocated her feelings (As per her "Oh, if only you were well and we could be together!" comments) she would immediately get angry and leave them for lying.
Except the one in physical therapy, who really did take that step out of love.
In Greys Anatomy, Izzie falls in love with patient Denny. She nearly got fired, she stole a heart, he died and then she quit. After that they continued their relationship through cancer-hallucinations. It wasn't really a success.
Alex slowly falls for Jane Doe/Eva as she recovers in the hospital with no face, no memory, and a baby. He eventually does the noble thing and lets her go back to her husband. A few months later, she comes back to the hospital and they have sex.
Teddy feels sympathy for Henry, a patient who has a chronic illness and marries him because he has no insurance and can't afford treatment. She eventually falls in love with him and agrees to a romantic relationship, but he dies soon after.
Interestingly enough, the actor playing Henry also appeared on Scrubs as Sean who was a Patient of the Week that Elliot starting dating, though he only asked her out once he was discharged and didn't have any significant health problems.
In Ugly Betty, after suffering a heart attack Ignacio falls for his house nurse, Elena, with her returning the feelings. This sparks feelings of resentment and anger from Hilda who views the relationship as inappropriate.
Invoked by name on Royal Pains by the lead, after the young girl he saves falls for him.
Spoofed in Friends, where two of the girls fall for a coma patient and argue over him. When he woke up they both try to impress him by their excessive Florence-Nightingale-efforts. Averted in that when the coma patient wakes up, he's ungrateful for what Phoebe and Monica have done and they both pseudo dump him.
a young, teenaged cancer patient develops a bit of a crush with Dr. Lewis.
One case was when Tony fell for his kid patient's mother.
Wilson from House. Cameron is also prone to this with "broken" people, like her ill ex-husband and House himself.
Lois and Clark had Lois's amnesia therapist fall for her. In a rather extreme version of this trope, rather than help her recover her memories, he actively sabatoges her attempts to regain her memory (and her relationship with fiance-Clark) and instead hypnotises her to leave with him.
Happens all the time in Farscape, e.g. Crichton/Aeryn (either way) or Zhaan to D'Argo.
In WKRP in Cincinnati, Andy is knocked out by...well, by nothing in particular when a tornado hits Cincinnati. Jennifer runs to give him mouth-to-mouth, which quickly turns into...something else.
Referenced on Seinfeld when one of Elaine's boyfriends is in the hospital. George mistakenly calls it the "Clara Nightingale Syndrome," confusing Florence Nightingale with Clara Barton.
Arguably Chuck and Eva on Gossip Girl, depending on whether or not you believe he had genuine feelings for her.
In Doctor Who, Martha having to perform CPR on the Doctor might have helped develop her crush for him in series 3. Of course, that kiss... Er, genetic transfer didn't hurt, either.
Elliott: It's totally inappropriate to jump a patient isn't it?
Carla: Oh yeah.
Elliott: Have you ever done it?
Carla: Tons of times.
In The Time Tunnel, Tony falls in love with Sarit, who is administering medical care to him when recovering from an experience on the rack.
Subverted in "The Nurse Who Loved Me," by Failure and A Perfect Circle. The speaker is a man in a hospital who imagines that he and his nurse share a romance because she brings him drugs. However, it's clear that he only cares about the drug high, and the romance she feels toward him is just an invention of his addled mind. He's lying face-down on the floor throughout the song and occasionally lapses into complete gibberish.
In the Ed Sheeran music video for "Give Me Love" the girl chosen to be a cupid and therefore forever alone has a Maybe Ever After moment with a paramedic who revives her.
Used as an important backstory component in Dino Attack RPG. Kate Bishop only exists because her mother had an affair with a young Alan Pierce while recovering from injuries suffered by a hit-and-run driver.
Once On This Island is a definite example. Ti Moune follows Daniel back to the world of the Grand Homes and nurses him back to health after he gets hit by a car. During the songs "Some Say" and "Pray: Reprise", their relationship grows, and Daniel falls in love with the already smitten Ti Moune.
The Devil Social Link in Persona 4 explores this through Sayoko, a Hospital Hottie who deals with this from patients all the time. She claims not to take such confessions of love too seriously, commenting on the circumstances as well as how, eventually, they all leave.
Note that she repeats the pattern: She's a White Mage, Cecil's a Paladin. Their quest to defeat Golbez, and then Zemus, cements their love. They produce Ceodore, a WhiteMagic Knight.
Final Fantasy VIII Raine falls in love with Laguna Loire this way; after he is nearly killed on a mission for the army, she spends six months caring for him until he recovers.
This is a popular matter within the Team Fortress 2 fandom. Although the Medic can heal someone from 1 HP to 150% health in a matter of seconds, he also follows them into battle, and the healing target is compelled to watch out for his Medic.
In Fire Emblem 7, Jaffar already had some degree of interest in Nino, but it started to turn into friendship (and love, if you get their A support) only when she nursed him back to health after he was seriously injured (and going against the law of the Black Fang, actually, since she should've given him a Shoot the Dog treatment). It was made even stronger when she begged him to spare Zephiel and he made a Last Stand for her.
In Guardians Crusade, after Knight finds a near-dead Nehani in the forest and nurses her back to the peak of health, she falls in love and becomes his sidekick ala Peter Pan and Tinkerbell. At the end, she gets a Relationship Upgrade and becomes his wife.
The early interactions with Esperia in Eien no Aselia involve this as Esperia nurses Yuuto back to health and teaches him the local language. She falls in love quickly enough, but since it's a multiple endings kind of thing she's left in the position of secondary heroine to Aselia.
In Heavy Rain, almost every time Madison Paige meets Ethan Mars, she's either bandaging his wounds or trying to help him find his son while planning to write a story about him. Her desire to help him eventually turns into love. Depending on player decisions like whether or not Ethan will forgive her for trying to get a story out of his ordeal, they can even end up married.
The podcast The Bugle, in its recurring "Hotties from History" segment, insists that the actual Florence Nightingale was the ultimate example. Her name is occasionally used as a byword for sexiness in general.
Doubly funny if you realize that Florence Nightingale's diaries are full of her varied romantic adventures with women - and the only direct nursing experience she had was with male patients.
This is pretty much how Jay Sherman of The Critic met his first wife, who fell for him while he was in a full body cast and unable to speak. Her love faded away as soon as the bandages came off, though.
In the Disney version of Beauty and the Beast, Belle treats the Beast's wounds after he saves her from the wolves. They don't kiss, but the scene is where their relationship takes a turn for the better.
On King of the Hill, the only woman Cotton ever really loved was a Japanese nurse who took care of him after the war. Years later he returns to Japan to find her again and discovers that they had a son after he was forced to leave.
Worth noting that his second wife was also his nurse decades later in the VA hospital.
A light version of this could be interpreted to happen in Avatar The Last Airbender: the first thing Aang sees after waking up in the iceberg is Kataraís face as she helps him up and makes sure heís okay; this scenario is repeated after he is exhausted from entering the Avatar State and after he is nearly killed by Azulaís lightning and Katara uses her vial of spirit water to heal him; Aang dutifully spends the series falling head-over-heels for her.
There have been a number of documentaries made about a group of badly disfigured British WW 2 veterans who married their nurses. The long time the soldiers spent in recovery let the nurses get to know them and see past the scars. In this case, too, the nurses were explicitly told that crossing the nurse/patient barrier was acceptable under the circumstances, if they wished to - the men had been so horrifically disfigured that it was thought that no other woman would marry them. Pretty much any war up to the 1991 Gulf War (nursing stopped being viewed as a female-dominated field around that time, not earlier) has had a movie or five built around this trope.
The patient falling in love with the doctor variation occurs quite often for psychiatrists, due to the fact that a therapist-patient relationship can last years and that the patient is spilling out his soul for most of that relationship.
Nightingale herself took a very dim view of this, nurses who did it, and hospitals that tolerated it.
In an attempt to avert this trope, when Clara Barton formed a nursing service during the American Civil War (which eventually became the American Red Cross), she specified that all the women who applied as nurses be "at least 30 and plain-looking."
Red Pollard, Seabiscuit's most famous jockey, was crushed by a horse and spent months in the hospital trying to re-grow a rib cage. He later married his nurse.
Stephen Hawking married his personal care assistant. Sadly, it didn't last.