"A kiss triggers a cascade of neural messages and chemicals that transmit tactile sensations, sexual excitement, feelings of closeness, motivation and even euphoria.
Kisses can convey important information about the status and future of a relationship. At the extreme, a bad first kiss can abruptly curtail a couple’s future.
Kissing may have evolved from primate mothers’ practice of chewing food for their young and then feeding them mouth-to-mouth. Some scientists theorize that kissing is crucial to the evolutionary process of mate selection."