Why guys like breasts is what you asked there, predess? It's probably because of violence, guys like to squeeze them, to feel in control, to feel power, which raises the adrenaline, and at the same time the excitement. And since it's a subconcious thing, it's pretty hard to research anyway.
Just like asses. Why the hell do i like asses? I mean, SHIT comes through there... Violence probably, just violence, it occupies the most space in our brains.