According to a report from Live Science, we have ancient Greece to thank for the tradition of kissing under the mistletoe, as the ancient Greeks associated it with fertility. The plant was used during the feast of Saturnalia, and later on in marriage ceremonies.

What’s really interesting is that in the Roman era, enemies at war would actually reconcile under mistletoe because they felt it signified peace.

Source: https://www.distractify.com/p/why-do-we-kiss-under-mistletoe