According to Norse mythology, Baldur dreamed about his own death and fell into depression, believing that every living thing was out to kill him.
He was so beloved by the other gods that they resolved to protect him from the dangers of the world.
His mother Frigg, goddess of love, marriage, and beauty, took an oath from all of the elements, plants, and animals that they would not harm Baldur, but neglected to extract that same promise from the young and insignificant mistletoe.
When Loki, god of mischief, discovered the oversight, he made a poisoned dart with the mistletoe and used it to kill Baldur in front of all the gods. The berries became the tears of his mother, and it was decreed that henceforth, mistletoe would bring only love into the world and not death.
It was placed under doorways so that it would never be overlooked again, and any two people passing under the plant would exchange a kiss in memory of Baldur.