Let's presuppose that, historically, Achilles was actually a fighting demigod whose ankle was weak because his mother gripped him by it while submerging him in a magical immortality stream. Everyone on board with that? Great. After Patroclus was slain by Hector, Achille's grief was so great that he wasn't able to dispose of the body until Patroclus's ghost (this is a real story that really happened with real people) came to him and told him to let him go. Achilles then killed Hector, and, rather than returning Hector's body to the Trojans, dragged his corpse around behind his chariot. For a while, anyway.
At least in Shakespeare's version of their tragic love story (adapted from Plutarch) the two die imagining their reunion in the afterlife. That sounds awful. Don't make suicide pacts. Still, we wouldn't mind having someone say that "age cannot wither her, nor custom stale / Her infinite variety: other women cloy / The appetites they feed, but she makes hungry / Where most she satisfies" to us. We're in favor of that.
Heloise was one of the foremost female intellectuals of her time, and Abelard was a renowned philosopher. Shortly after he began tutoring her they fell in love, and, while Abelard wanted a secret marriage, Heloise said she'd prefer to remain his mistress. They had a child named Astrolabe. Seemingly, Heloise's uncle didn't find this as a cool and modern and progressive as we did, because he castrated Abelard. Abelard then went off to be a monk, and Heloise became a nun, but they continued to exchange love letters until they died.
Dante first met Beatrice when he was 9 and she was 8. He was so entranced with her that he continually wrote her poetry, despite not seeing her again for the next nine years. When he did see her, she spoke to him, and afterwards he went to sleep contemplating their meeting. He claims that during that night he had a dream that served at the inspiration for the Divine Comedy.
Elizabeth betrayed her father's wishes to run away and elope with Robert. Despite the fact that she was the more successful writer of the two, Robert never minded, perhaps because Elizabeth claimed that his letters - of which there were many - "threw her into ecstacies."
While Marilyn Monroe's second marriage to Joe Dimaggio was very brief, they remained friends throughout their lives. Following her suicide a half-finished letter was found in Marilyn's apartment addressed to DiMaggio stating, "If I can only succeed in making you happy, I will have succeeded in the biggest and most difficult thing there is – that is, to make one person completely happy. Your happiness means my happiness, and..." Following her death, DiMaggio never remarried and continued to have roses sent to her grave twice a week until he died.