Breasts: Guys it's a good thing to love them

Jokes about breasts, and men looking at breasts, are so common place that they have become a cliché. How many times have you seen a man talking to a curvaceous woman only to have her point to his eyes and say "Hey, buddy, up here!"? It's funny  because it's true. The male eye does have a way of drifting south. But why? Why are heterosexual men so fascinated by women's breasts? Being an educated man (of sorts) I would like to propose an answer.