When oxytocin is released at the baby’s instigation, the attention of the mother focuses on her baby. The infant becomes the most important thing in the world. Oxytocin, acting in concert with dopamine, also helps imprint the newborn’s face, smell and sounds in the mother’s reward circuitry, making nursing and nurturing a feel-good experience, motivating her to keep doing it and forging the mother-infant bond. This bond is not only the most beautiful of all social bonds, it can also be the most enduring, lasting a lifetime.
Humans are among the very rare animals that have sex face-to-face, looking into each other’s eyes. Some believe this quirk of human sexuality has evolved to exploit the ancient mother-infant bonding brain circuitry as a way to help form bonds between lovers. In other words, men can make themselves more desirable by stimulating a woman’s breasts during foreplay and sex, provoking the release of oxytocin. Evolution has, in a sense, made men want to do this.
Read more on the subject here: http://www.livescience.com/23500-why-men-love-breasts.html