What is Oxytocin?

Oxytocin is a mammalian hormone that also acts as a neurotransmitter in the brain.
In women, it is released mainly after distension of the cervix and vagina during labor, and after stimulation of the nipples, facilitating birth and breastfeeding, respectively.
Oxytocin is released during orgasm in both sexes.

(Which may be why we are not very good at unemotional one-night stands!)