HARRY:Because no man can be friends with a woman that he finds attractive. He always wants to have sex with her.
SALLY: So, you’re saying that a man can be friends with a woman he finds unattractive?
HARRY: No. You pretty much want to nail ‘em too.
SALLY: What if THEY don’t want to have sex with YOU?
HARRY: Doesn’t matter because the sex thing is already out there so the friendship is ultimately doomed and that is the end of the story.
When you slowly start hating someone you were friends with.