Apollo and Daphne - This is a myth with the moral being not to mess with Cupid. When the master archer Apollo insulted Eros (the Greek name for Cupid) for messing around with serious war weapons, the boy shot Apollo with one of his own arrows, causing him to fall in love with the nymph Daphne. This daughter of a river god was a sworn virgin, and she rejected the advances of the deity. He pursued her relentlessly, however, and as this was in an era prior to restraining orders, she had her father turn her into a laurel tree.
Echo and Narcissus - If Whitney Houston is correct that learning to love yourself is the greatest love of all, then I suppose Narcissus would have been the greatest lover in history. As for Echo, she was a nymph who distracted Hera while Zeus had an affair, so the goddess punished her by making her unable to speak except for repeating what other people say. I believe Ovid was the first to combine these two characters into the same myth, having Echo fall in love with Narcissus, who of course rejected her advances. She ended up wasting away until all that was left was her repeating voice. Narcissus himself eventually died while staring at his own reflection, with the narcissus flower blooming where he died.
Freyja and the Dwarves - This one is really only tangentially related to my main subject, but it's so messed up that I can't help including it. Turning from the Olympian world to the Norse Aesir, Freyja found out about a dwarf-made necklace called the Brisingamen that she desired so greatly that she agreed to have sex with all four of its dwarven makers. When Odin found out about Freyja's harlotry, he had Loki take the necklace away from her, which the trickster accomplished by turning into a flea. Freyja demanded the Brisingamen back, and Odin conceded, but only on the condition that she cause war between two earthly kings. That seems like a bit of a non-sequitur, but maybe the point was that starting warfare would be devastating for a goddess of love.