Animorphs: However much Jake and Cassie loved each other - and it was a lot - being together after the war wouldn't have been good for either of them. Despite that, being apart hurt them as well because of how much they cared about each other. Also, Tobias loved Rachel so much that her death destroyed him.
This is far and away one of the oldest, most universally recognized tropes in existence. A perfect example of it is found in the 12th century Persian epic, Layla and Majnun. Boy meets girl, boy loses girl, boy loses mind. It's not a cheerful tale.
That epic inspired the classic German ballad of Heinrich Heine, "Der Asra". "...who die if they love."
Edgar Allan Poe was a very firm believer in this. He thought the best theme to write about was the death of a beautiful young woman, often leading to the protagonist's descent into madness.
Given the things that happened to women in his life, this isn't really surprising.
For those who don't know, tuberculosis killed every woman he lovednote Except for his first girlfriend; She died of brain cancer.. It killed his mother when he was young, two women that he was courting, and his wife. It's often speculated that the disease, which involves a great deal of blood loss, was the titular "Red Death" in his short story, "Masque of the Red Death".
Yuan, Lord of City Europe in David Wingrove's Chung Kuo series, falls for the wrong woman. Although the stage is set for serious tragedy, he manages to pull out of it.
Zoey ends up falling for three guys simultaneously (ish). It ends badly when she thinks her own-age vampyre boyfriend Erik is dead, flees to the hot-but-older Loren for comfort, and ends up losing her virginity to him. Proving that Joss Whedon is not the only one who can be incredibly cruel to his characters, the following things happen in quick succession: it breaks her bond with her human boyfriend Heath, Erik walks in on her and Loren Kissing, and it turns out Loren was in cahoots with the villainess and only using Zoey. And then Loren is brutally murdered. Ouch. Poor Zoey. And it gets worse for her: the guy she displays a brief romantic interest in in the fourth book promptly dies, only to be resurrected as a bizarre zombie-vamp hybird controlled by her archnemisis. Zoey has a bit of a Cartwright Curse, it seems...
At the end of Burned between Stevie Rae and her consort Rephaim. The returning of Zoey's and Kalona's souls to their respective bodies serves to keep the two apart.
In the Warrior Cats series, falling in love is likely to get you broken-hearted, insane, and/or killed. Bluestar wound up with all three because of her relationship with Oakheart. Some lucky couples just end up getting their kits killed off. Yes, you heard right, they are the lucky ones.
Though The Engineer Trilogy as a whole plays this trope catastrophically straight, there is a scene lampshading it in the second. Miel Ducas, a powerful noble wounded in battle, hallucinates that he is having a conversation with Death. When Death points out that Ducas is not very grateful for having everything a man could ever want, Ducas replies that he is the poorest man in the world because no woman he ever loved returned his affection. Death explains that love is nothing more than a trick meant to override humans' free will in order to reproduce, and that Ducas might as well be upset that he never contracted diphtheria.
Yes, I was a fool, but I was in love, and though I was suffering the greatest misery I had ever known I would not have had it otherwise for all the riches of Barsoom. Such is love, and such are lovers wherever love is known.
The Duel Of Sorcery series: Serroi and Tayyan are a couple at the beginning; then Serroi freaks out and effectively gets Tayyan killed. Rane and Merralis were a couple; Merralis contracted a terminal disease, and Rane is still mourning years later. Michael's boyfriend is beaten to death in front of him. Serroi and Hern have a thing for a while...but then, Serroi gets turned into a treein order to defeat Ser Noris. (She does get better in the follow-up trilogy, but Hern's long dead by then.)
Richard and Alec in Swordspoint and The Privilege of the Sword'', to the point where their relationship is such a mess that neither they nor the reader can figure it out.
A recurring theme in sonnets, especially those from the Renaissance.
The Great Gatsby could be argued to have love kills, and causes killing. It's arguable because actually being "in love" strongly tends to make the lovers happier - it's the fallout on everyone and everything else around them that actually brings the pain.
In L. Jagi Lamplighter's Prospero Lost, Ferdinard speaks eloquently of having suffered torments for the pure love of a chaste maiden.
Anne of Green Gables: Poor Gilbert Blythe lives this trope for over ten years; he falls in love with classmate Anne Shirley the day she smashed his slate over his head, but the reason for the smashing — Gilbert calling her "Carrots" and teasing her for her red hair, causing her to resent him for many years. When they finally become friends, Gilbert bides his time. He proposes, she turns him down. He proposes again (a couple years later), and she shoots him down again. It takes a near-death experience with typhoid fever for Gil to get his girl.
Many romantic relationships suffer from this in A Song of Ice and Fire as par its Deconstructionalist nature, but no other character can claim that they've been through the worst of the Love Meat-Grinder than Petyr "Littlefinger" Baelish, who dared to fall in love with a girl above his social status. For his romantic idealism he was almost killed, possibly raped by the younger sister of the girl he loved, who then got pregnant and was tricked into aborting the child by her father, and sent packing from the home of his foster family, the place where he had grown up, back to his dreary homeland where no one knew him and he was mostly likely doomed to live out a lonely exile. The whole experience left him so severely unhinged that he became a Magnificent Bastard and started a continent-wide civil war to get back at his childhood crush.
Harry Dresden's romance life is an exercise in pain. Of the women he's been involved with or who he has strong feelings for/who have strong feelings for him, one gets turned into a vampire, has his child, and gets sacrificed, another is the copy of a Fallen Angel living in his head that kills herself to save him, and another is only in love with him because The Mole was mind-controlling her. Murphy still cares for him, but her reward for her affections is learning Harry is now dead. And then there's Molly, who goes from teenage crush on Harry to honestly being in love with him, but carrying all the baggage of his death as well and going almost completely nuts thanks to Black Magic and mental trauma.
The Picture of Dorian Gray features Basil who has to watch Dorian Gray, the love of his life and his greatest inspiration, descend deeper and deeper into corruption against his wishes and ultimately gets murdered by him. And then there's Sybill Vane who takes a fatal dose of poison right after Dorian cruelly dumps her.
Poor, poor Severus Snape. Spent the last years of his life trying to protect the son of the woman he loved — the same woman who rejected him for consorting with the Death Eaters (which was his fault entirely), and the same woman he all but killed. To make it worse, the guy she married was his worst enemy (who was reformed by that point). What makes it so painful is that their son is the spitting image of his father, yet his most distinguishing feature is his mother's eyes, reminding Snape of his failures and the fact that he Did Not Get the Girl. It's implied that the real reason why Snape projected James onto Harry is because if he saw anymore of Lily in him, he'd snap from pure grief at the reminder — it isn't until he's dying that he finally sees Lily in Harry.
Though if Snape had it bad, Albus Dumbledore had it worse. The love of his life was the Dark Lord Gellert Grindelwald, who manipulated Dumbledore into helping him create plans for a world where wizardkind subjugated muggles "for the greater good". Dumbledore, already resentful over how his ambitions and dreams were dashed away to take care of his younger siblings (and due to the actions of three muggle boys, no less), was so enamoured with Grindelwald that he went along with this insanity, setting off a chain of events that led to a three way duel between the Dumbledore brothers and Grindelwald, culminating in the death of their sister. After of which, Grindelwald promptly fled the scene and abandoned Dumbledore. Unsurprisingly, Dumbledore gave up on romantic love after that, though that didn't stop fate from twisting the knife further by forcing Dumbledore to face off with Grindelwald in an epic duel forty-six years later, with the fate of the entire world on the line. One has to wonder if the real reason why Dumbledore espouses about The Power of Love so much is because love has been such a bastard to him over the years.
In The Hunger Games Katniss isn't so much deciding between two boys (it's made clear halfway through the book that introduces the triangle which guy she's fallen in love with) as it is her deciding whether or not she dares to love. She's terrified of losing people close to her heart and her only real experience with genuine love is the love between her parents. They loved each other a lot but then her father died and left her mother a shell of a woman. Katniss is determined never to care that much about another person which is why she keeps resisting/ignoring her own feelings. Then comes the third book and Peeta, the guy she's fallen in love with, gets captured by the Capitol and is tortured, mind raped and turned into a weapon against her. Cue all that emotional pain she was trying to protect herself from.
The House of Night: Does it ever. Oh, does it ever. The end of "Chosen," anyone? Or "Tempted?" Though, considering that she was juggling three boyfriends at the time, the impact may have been lessened.
At the End of Burned between Stevie Rae and her consort Rephaim. Zoey and Kalona's souls return to their bodies, thus keeping the two apart.
The Silmarillion: Beren and Lúthien go through absolute hell for one another. Beren is a mortal Man, and Lúthien is the daughter of the Elven king Thingol and the Maia Melian. After eventually losing his father and all his kin fighting Morgoth, Beren wanders into Thingol's kingdom, where he has the gall to fall deeply in love with Lúthien. Thingol does not take this well, and sets him an Impossible Task: retrieve a silmaril from Morgoth's crown. Along the way, Beren and his companions are captured, tortured, and killed one-by-one by Sauron until Lúthien shows up and humiliates him. Together, Beren and Lúthien succeed in recovering one of the gems, but then Beren loses his hand (with the silmaril clutched inside it to the wolf Carcharoth. And then Beren dies helping slay the wolf to get it back. Then Lúthien dies of grief, and is faced with an eternity of being separated from her love because Men are not allowed to remain in Valinor upon death, while Elves are denied to pass wherever it is the souls of Men go. Her grief is so profound that her song of her fate manages to move the otherwise pitiless Mandos, who asks Manwë to intercede on their behalf, and the lovers are sent back to live again together. And this is pretty much the only happy story in The Silmarillion.
19th century writer Higuchi Ichiyō's works are rife with this. Nearly all of her protagonists are young women, most are in love and none of them ever end up happy. If they aren't in love, they are married to a man they don't love and end up depressed, poor, and/or dead one way or another, with no way to escape their fate. She herself considered that in those times, women had very little power over their lives, so her writings are… not very optimistic overall. It doesn't help that she herself had been hurt by her unrequited love for her mentor in her first years of authorship.
The Unexplored Summon://Blood-Sign: Between Kyousuke and his first love, the White Queen (an all-powerful Eldritch Abomination). When he first summoned her to Earth, she was childishly innocent and had no inclination to hurt anyone. But others sought her power, which started a chain of events that caused the Queen to become violent and kill many people, including most of Kyousuke's surrogate family. Then an army of summoners attacked her in order to stop her rampage, while an unconscious Kyousuke was next to her. She was faced with the decision to either give up and let Kyousuke be killed in the crossfire, or to slaughter the army to save him. She chose the latter, knowing that Kyousuke would always hate her for it. In the present, Kyousuke is driven to save anyone in need to atone for his role in the Queen's actions, while the Queen will go to any lengths to make him hers again.