When it comes to matters of sex (often an important piece of the romantic love puzzle), a hormone called oxytocin plays a major role. Oxytocin is released, which helps bond the relationship plays a critical role in everything from childbirth to breastfeeding, but it's also a crucial bonding component in sexual activity, erection, ejaculation, orgasm and more.
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, oxytocin helps initiate an emotional bond — the more sex, the greater the bond.
Oxytocin has also been shown to play a role in fidelity: One study found that men in monogamous relationships who were administered oxytocin maintained a greater distance between themselves and attractive females than men who received placebos and men who were not in relationships. 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. Fisher believes that oxytocin and vasopressin interfere with the dopamine and norepinephrine pathways, which might explain why passionate love fades as attachment grows. But passionate love doesn't necessarily have to burn out, either. According to a study published in the journal Social Cognitive and Affective Neuroscience, the fMRI brain scans of long-term married individuals and individuals who had recently fallen in love revealed both groups demonstrated similar activity in specific brain regions. Researchers focused on one brain region specifically: the dopamine-rich ventral tegmental area (VTA) and concluded that "for some individuals, the reward-value associated with a long-term partner may be sustained, similar to new love, but also involves brain systems implicated in attachment and pair-bonding." One potential factor that contributed to the enduring passion of the long-married individuals? Sex (duh). Participants in long-term romantic love reported high sexual frequency, which is associated with activation of another part of the brain called the posterior hippocampus.
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." High endorphin levels are released during exercise, fear, love, music, chocolate eating, laughter, sex, orgasm, etc. Increased levels of endorphin inhibits pain in the body and reduced levels of endorphin inhibits positive feelings.