I had a chat with my now-ex about this a while back:
ex: i never understood what it is with men and boobs
me: it's something in the shape, motion and illicit, concealed nature of the aforementioned bags of flab that gets us, i think
me: and that's as much insight into the male psyche as you're getting
me: oops, i seem to have scared them all away
ex: no you haven't scared us, just trying to understand it, but i have come to the conclusion that men aren't supposed to be understood.
Other than that, I'd say it's the forbidden nature of the boobs, their tactile lusciousness, the sexual pleasure they give to our women, and the psychological associations given to them by our society. So, pretty much what the above responses have said.
Nonetheless, yay for boobs - one of the world's greatest things.