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.
In other words, men can make themselves more desirable by stimulating a woman's breasts during foreplay and sex. Evolution has, in a sense, made men want to do this.
Attraction to breasts "is a brain organization effect that occurs in straight males when they go through puberty", Young told Life's Little Mysteries. "Evolution has selected for this brain organization in men that makes them attracted to the breasts in a sexual context, because the outcome is that it activates the female bonding circuit, making women feel more bonded with him. It's a behavior that males have evolved in order to stimulate the female's maternal bonding circuitry"