It all begins in infancy. Breast feeding is the most soothing, pleasurable experience an infant has in its existence. It's actually the food that is pleasurable, but the experience of the food comes to be associated with the closeness to the breast. My sister just gave birth, and I visited her for two weeks - it's amazing how much her infant loves her breasts. Even after he was switched to bottle-fed, he will sometimes push the bottle away and try to push his face against her breasts, attempting to suckle on them through her shirt.
It's no coincidence that straight males tend to adore breasts which look milk-filled. "Sagginess" is looks like a breast is flat and unfulfilling, while "perky" breasts look like they are ready to provide sustenance. It's no wonder that adult males still enjoy sucking on the breasts of women, even though they serve no sexually reproductive purpose. It's actually my opinion that breasts are the primary sexual object in normal human growth, and all other attractions deviate from this point.
Heteronormativity: the post.