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.
Vasopressin, an antidiuretic hormone, is another chemical that has been associated with the formation of long-term, monogamous relationships (see "Are We Alone in Love?"). Dr. Fisher believes that oxytocin and vasopressin interfere with the dopamine and norepinephrine pathways, which might explain why passionate love fades as attachment grows.
Endorphins, the body's natural painkillers, also play a key role in long-term relationships. They produce a general sense of well-being, including feeling soothed, peaceful and secure. Like dopamine and norepinephrine, endorphins are released during sex; they are also released during physical contact, exercise and other activities. According to Michel Odent of London's Primal Health Research Center, endorphins induce a "drug-like dependency."