not only breasts (which are wonderful), but also nipples (see:
What's in a nipple? (NSFW)).
But I'm not sure.. I blame Freud and Oedipus.
Freud would probably say: "You suck on your mom's boobs for the first few years of your life, so its something familiar to you. Sucking later on becomes sexual, so its familiar and sexual. And every guy wants to screw their mom and kill their dad anyway, so a socially acceptable substitute is finding mommy-like features in the person you want to screw." (my friend, masquerading as freud)
That sounds good enough to me, and does work, come to think of it, my g/f is shaped kind of like my mom, but I didn't really need that mental image at the moment.
oh well.. chew on that.