If being completely honest, let's face it, it happens to every man when he sees a woman his glance stops for a moment on her breasts. It's like an instinct, in male veins, and we just can not control ourselves in such moments, just unconditioned reflex. As all our reflexes and instincts, this one has its own logical explanation as well.
We represent this type of species, that from the moment we born we have some connections to female breasts. Men are the only male mammals fascinated by breasts in an intimate context. Women are the only female mammals whose breasts become enlarged at puberty, independent of pregnancy.
Female breasts have also quite important role in love life.
What are the motives of such attraction? I guess, we should count the fact that a connection between mother and her child starts from breastfeeding, but how? So, here's the answer, nipples are stimulated during breast-feeding, a neurochemical oxytocin – know as the ‘love drug’ – floods the brain, so this may cause a special bound between partners as well.
That is why women like breasts stimulation, and men like to indulge them.
So we can suggest, that men are atrracted to breasts because of an opportunity of bringing pleasure to a partner, and if you think about it... it's understandable why men like big breasts. Larger breasts - larger zone of stimulation.
Still, none of this can justify almost grasping male behaviour, sometimes.