Kissing under the mistletoe is thought to spring from Frigga, the Norse goddess of love, who was associated with the plant.
The Druids are also partly responsible for the use of mistletoe at Christmastime. They regarded the mistletoe as sacred, made certain that it never touched the ground, and dedicated it to the Goddess of Love, which explains the kissing that goes on under it.
Originally, when a boy kissed a girl, he plucked a berry from the cluster and presented it to her. When the berries were gone, so were the kisses.