Oxytocin. 

Oxytocin is a powerful hormone. When we hug or kiss a loved one, oxytocin levels drive up. It also acts as a neurotransmitter in the brain. In fact, the hormone plays a huge role in pair bonding. Prairie voles, one of nature’s most monogamous species, produce oxytocin in spades. This hormone is also greatly stimulated during sex, birth, breast feeding—the list goes on.