In romantic love, when two people have sex, oxytocin is released, which helps bond the relationship. According to researchers at the University of California, San Francisco, the hormone oxytocin has been shown to be "associated with the ability to maintain healthy interpersonal relationships and healthy psychological boundaries with other people." When it is released during orgasm, it begins creating an emotional bond -- the more sex, the greater the bond. Oxytocin is also associated with mother/infant bonding, uterine contractions during labor in childbirth and the "let down" reflex necessary for breastfeeding.
interesting read
http://people.howstuffworks.com/love7.htm