The Ancient Celts used mistletoe as an animal aphrodisiac, or more specifically, to increase the fertility of sheep. Such became the mythic power of mistletoe that in addition to bringing a lamb-ful spring, mistletoe was hung over doorways to ward off fire, lightning and evil spirits. But despite its protective properties, mistletoe couldn't shuck its fertile past, and even though it was hung in people's doorways, it seemed as if something romantic should occur in its presence.
Thus the kissing.
Did you know that mistletoe's power runs out? Every time a man steals a kiss under the mistletoe, he must pay by plucking one of its berries. When the berries are gone, no more smooching.