Depends on the version. In some, the Joker is nutters but not a complete psycho: actually a good match for a bad girl. In others, he's a real whackjob but she loves him and doesn't want to leave. Not sure I've seen any where she wants to leave but can't.

I do suspect that a key motive for Harley picking on Batman is jealousy. That girl likes attention and Bats is always distracting her boyfriend.