Oxytocin

A hypothalamic controlled neurohormone released via the posterior pituitary gland, facilitating social bonding, lactation and maternal behaviour. See also vasopressin, which is also the neurotransmitter involved in the formation of close pair bonds.

Reference:

Bear, M.F., Connors, B.W. & Paradiso, M.A. (2001). Neuroscience: Exploring the Brain. (2nd Edition). Baltimore, MD: Lippincott Williams & Wilkins.