After TOMI I think the Voodoo Lady has something to do with it. But in chapter 5, she hinted that she doesn't understand/can't control love, as it's a higher power than voodoo (can't remember it word for word, but it's along the lines of that). So I'm not completely convinced that is why he loves her.
I'm gonna go with the theory that he was originally one of the many local pirates that liked her (like the ones Elaine mentioned in SOMI that she told Guybrush about). I'm assuming it was for her looks, and quite possibly her personality. As LeChuck is a very obsessive character, this may be why he didn't accept her rejection.
Maybe something happened between them before SOMI for all we know; the idea of her having something to do with the secret of Monkey Island is a nice theory too.
I'm not crazy; I'm sanely-challenged.