Love is opium

The brain releases endorphins when we're injured to reduce pain, and also when we are with people we love, or when someone we love touches us. A lot of neuroscientists think we probably become addicted to or dependent on people in a similar way to heroin or morphine addiction, too. People who are attached to each other develop a social dependence on each other that's based in a physical dependence on brain opiates.

Temple Grandin: Animals in Translation