I'm assuming you mean click... if that's the case, I think they bond because they're both outsiders.  Edward is an outsider in the world of humans and within his own family because he doesn't have a permanent partner.  Bella's an outsider, not only because she's new, but because she doesn't necessarily get excited from all of the traditional high school stuff her friends at Forks HS does.  They're both lost within their own social clique.