Though there are 700 hypotheses on the origin of hanging mistletoe over a door, here’s my best interpretation:
(First, assume any plant that’s green all winter gets special attention in the north. Mistletoe is one of those.)
Now assume it’s the Dark Ages and there are no police. People, being people, are going to peeve each other, take one another’s sheep, slay one another’s relatives, etc.
There are also no Hallmark cards to send, with pretty writing inside that says, “All right, forget about the five chickens you owe me. I miss having that beer you make.”
Come the mid-winter festivals of “oh my god will this this darkness ever end,” people would bring green plants indoors to remind themselves to not kill themselves just yet. And there being no TV to watch, they invited the neighbors in for some story-telling.
If neighbors were slow to materialize, paralyzed by self-consciousness over stolen goods or murdered relatives, a signal was needed to indicate that, today at least, the need for company is greater than the need for vengeance. Remember that reading was practiced mainly among monks. So in Scandinavia, up went the mistletoe over the door–the exterior door, so that you knew from a distance you would be safe.
So says my favorite folklore: All who entered were assured of forgiveness.
The kissing? Kissing was not always romantic and spitty. Consider the Pope’s ring. Jesus and Judas. Kissing used to be really big in the sorting out of status. And as a symbol of allegiance. To kiss under the mistletoe was actually redundant: Both signaled that you were on the same team.
Today, anyway. Winter is long up here on the shoulder of the planet. It wouldn’t hurt to keep that mistletoe up for a while.