Many trace this tradition back to Norse Mythology, to a story about the god named Baldur. In the Story, his Mother casts a powerful magic spell to make sure no plant grown on Earth could be used as a weapon against her son.
The Sneaky mistletoe avoided her spell, because it was grown out of a tree’s branches, not out of the earth!
There was a devious fella named Loki who, when realizing this, made a spear out of Mistletoe and he killed Baldur with it.
Surprisingly, Baldur’s magic momma declared the Mistletoe to be a symbol of love after the death of her son and she promised to plant a smooch on anyone who walked underneath it.
Historians who ponder such important things seem to be baffled how this mythological tale somehow evolved into its modern-day tradition.