2 answers :

If you want a scientific answer, if you go back to the stone age and primitive man, large breasts were a sign of fertility and the ability to look after children, so would make a good mate. As time progressed this probably has been ingrained into guys perception and are now sub conciously doing what cave men did all that time ago.

And for the social answer. Boobs are just generally good ok? lol. I mean, well they just are. Its part of a women, women dont regulally flaunt them, so its sexual. Sexy things are good. Therefore boobies = good. QED

To quote Stewie Griffen "She has big breasts, which you can use as mountains for your matchbox cars or whatever grown ups do with bog boobs"