From The Science of Kissing: What Our Lips Are Telling Us, by Sheril Kirshenbaum
"A romantic kiss does nothing less than set off an avalanche of biological activity. During a passionate kiss, our blood vessels dilate; more oxygen is routed to the brain; our breathing quickens and becomes erratic; our cheeks flush; our pulse quickens; our pupils dilate; dopamine, serotonin, norepinephrine, oxytocin, and adrenaline levels spike, leaving our bodies awash in a chemical bath. In short, our biology seem to be hardwired to make kissing extremely pleasurable and, to some degree, addictive."
