Ok. Maybe love is too strong a word. But since the main fan base are typically teens and tweens, I can only assume it is due to the perfect guy of most girls' fantasies existing in the depths of that book.
Like seriously, which guy now would:
1. love a girl so much that he was willing to die for her (don't think about the stalking part. lol)
2. want to get married before making out/doing the dirty deed with her
3. refuse to have sex a second time after seeing the few bruises the girl had DESPITE the girl insisting that it was nothing and that she most certainly enjoyed the deed
4. be the perfect gentleman to the girl, even if the girl displayed blatant affection for another guy (LOL)
5. allow said guy to hug the girl because he could keep her warm
6. rich, know a million languages, is in almost every way an eligible bachelor (forget the fact that it's a vamp), and yet have eyes only for the girl
no such guys exist. and in fact, no guy with any sense of self-respect should tolerate a girl who shows such open affections for another. ha ha. but the point remains that edward cullen epitomizes most girls' dream guy.
point to note: i'm sooooooo not talking about robert pattinson. he's erm...ugly :p