In Norse mythology, Baldur (the son of Odin and Frigg) was prophesied to die. His mother (the goddess of love), secured an oath from all the animals and plants that they would not harm Baldur.
Unfortunately, Frigg forgot to talk to the mistletoe. Loki, the schemer, made an arrow out of mistletoe and this was used to kill Baldur.
In some versions of the story, the gods were able to resurrect Baldur from the dead. A happy mother, Frigg said that mistletoe really was a symbol of love and said she would kiss anyone passing underneath it.
From then on, mistletoe has been associated with fertility and vitality. The kissing tradition seems to have started in 18th century England, when it was permissible to steal a kiss from any woman standing under the mistletoe. To refuse, was of course, bad luck.
https://www.history.com/news/why-do-we-kiss-under-the-mistletoe
