Being physically affectionate not only feels good, but it also triggers our bodies to increase the amount of oxytocin produced. Oxytocin is often affectionately known as the "love hormone" because of that feel-good effect, but it also helps to promote a monogamous romantic relationship. And physical affection isn't limited to sexual intercourse -- even small gestures like holding hands or hugging count.
In addition to oxytocin, sex also increases the level of a specific neuropeptide -- a molecule in the brain that helps regulate areas such as learning, memory and reward -- called arginine vasopressin (AVP), which also facilitates bonding with your partner. Both substances play an important role not only in promoting intimacy between you and your partner but also in socialization skills.