Mistletoe has several origin stories. One story purports that the favorite Christmastime plant is used to remember the fallen son of the Norse God, Thor.
Baldur was uniquely paranoid. The poor guy awoke one day to think the entire animal and plant kingdoms had huge, murderous vendettas against him. In desperation, his mother and wife went on a quest to apologize to each and every animal and plant on planet earth. Unfortunately, the pair neglected to appease the mighty mistletoe and Baldur was swiftly killed by an arrow made from the wood of a mistletoe plant. According to this origin tale, the acts of placing Mistletoe above doorways and kissing beneath the plant, remind us to never forget.
Another origin tale goes back to ancient Druids who viewed the plant as a symbol of luck and fertility. The plant was believed to be magical and was hung above doorways. In any case, mistletoe is now commonly used to get and give kisses.
Washington Irving said it best: The “Young men have the privilege of kissing the girls under [mistletoe], plucking each time a berry from the bush. When the berries are all plucked the privilege ceases.”
This probably holds true to everyone who uses the Christmas plant exclusively for smooches.
For more info – Mistletoe: The Evolution of a Christmas Tradition