From Live Science: http://www.livescience.com/23500-why-men-love-breasts.html
Why do straight men devote so much headspace to those big, bulbous bags of fat drooping from women's chests? Scientists have never satisfactorily explained men's curious breast fixation, but now, a neuroscientist has struck upon an explanation that he says "just makes a lot of sense."
Larry Young, a professor of psychiatry at Emory University who studies the neurological basis of complex social behaviors, thinks human evolution has harnessed an ancient neural circuit that originally evolved to strengthen the mother-infant bond during breast-feeding, and now uses this brain circuitry to strengthen the bond between couples as well. The result? Men, like babies, love breasts.
When a woman's nipples are stimulated during breast-feeding, the neurochemical oxytocin, otherwise known as the "love drug," floods her brain, helping to focus her attention and affection on her baby. But research over the past few years has shown that in humans, this circuitry isn't reserved for exclusive use by infants.
Recent studies have found that nipple stimulation enhances sexual arousal in the great majority of women, and it activates the same brain areas as vaginal and clitoral stimulation. When a sexual partner touches, massages or nibbles a woman's breasts, Young said, this triggers the release of oxytocin in the woman's brain, just like what happens when a baby nurses. But in this context, the oxytocin focuses the woman's attention on her sexual partner, strengthening her desire to bond with this person.
But, I suppose the notion that men, like babies, love breasts begs the question: Why Do Women Have Breasts? Really, and why are they there at all?
In the primate world, plump breasts last only as long as breastfeeding does—except in humans. Women are busty all the time, even after menopause. According to some scientists, the trait is an evolutionary trick for snagging men and signals a woman’s ability to feed her children.
But no one can confirm an answer as to why women are the only apes with sizable chests .
Even though they appear full, a woman’s breasts are only filled with milk after a she gives birth. The rest of the time, they’re mostly made up of fat.
"Human breasts could therefore be a kind of biological deception," according to David P. Barash and Judith Eve Lipton at the National Sexuality Resource Center.
However, Barash and Lipton report that full breasts could just as easily signal the truth about a woman’s ability to store fat and her fertility. For example, flat-chested prepubescent girls are too young to bear children, and the sagging shrunken chests of older women may suggest they’re past their prime.
OK, I'm having some trouble getting my head around the notion of "biological deception". I really doubt that early man (as differentiated from premature men), or even the late ones, needed a "truth signal" about her fertility and child bearing prowess.
Breasts just are. They are forces of nature, like stars, tsunamis, puppies, bacon, and NASCAR. Do we really need to speculate about why we love them? Do we sit around and speculate why we like stars, tsunamis (well, maybe not so much love those tsunamis), puppies, bacon and NASCAR? We just do. Don't go mucking it up with "scientific" theories about why we love breasts. We just do.