Oxytocin is both a hormone and neurotransmitter, and is released in large amounts during childbirth, helps with lactation and pair bonding, has been associated with wound healing, and feelings of increased trust.
One study showed that Oxytocin affects social distance between males and females, with men in a relationship given an Oxytocin nasal spray choosing to increase their distance from an attractive woman at first meeting, compared to single men. The research suggested Oxytocin might help keep monogamous relationships together by promoting fidelity.