Oxytocin is a powerful hormone released by men and women during orgasm.
It probably deepens the feelings of attachment and makes couples feel much closer to one another after they have had sex. The theory goes that the more sex a couple has, the deeper their bond becomes.
Oxytocin also seems to help cement the strong bond between mum and baby and is released during childbirth. It is also responsible for a mum’s breast automatically releasing milk at the mere sight or sound of her young baby. (from Wikipedia)