A bit of a confusion that subject,really. One thinks that at one's advanced age one can't be surprised by discoveries in this er... area. But no, here it is:
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.The scientist quoted above is Larry Young, professor of psychiatry at Emory University who studies the neurological basis of complex social behaviors.
I guess that this "complex social behavior", what was for uncounted millennia fairly common, is elevated now from a simple popular practice to the level of scientific theory.
I submit that some psychiatrists should get out from their labs a bit more. And see more than just laboratory rats.
P.S. Actually not rats it is voles in this case. See this: