Mistletoe is found high in trees because birds eat the berries and spread seeds of mistletoe that sprout from bird droppings on tree branches. Don't think of this when kissing under the mistletoe.
Although the berries are, and always have been, poisonous the ancient Druids believed that the mistletoe was a sacred. They believed it had mystical powers to cure illnesses. It could ensure fertility. It was an antidote for poisons (remember it is a poison itself), and it could protect folks from evil, as wearing the cross on a cord around ones neck was first used to protect one from evil, the evil eye and diseases. This caused John Calvin to be opposed to wearing a cross for the superstitious origin of wearing one. Mistoetoe was also considered a powerful aphrodisiac.
Kissing under the mistletoe is a custom found in several cultures. In Greece doing so was part of a marriage custom. There are too many similar customs to related here.