In cultures across pre-Christian Europe, mistletoe was seen as a representation of divine male essence (and thus romance, fertility and vitality), possibly due to a resemblance between the berries and semen.
According to Pliny the Elder, the Celts considered it a remedy for barrenness in animals and an antidote to poison.
When Christianity became widespread in Europe after the 3rd century AD, the religious or mystical respect for the mistletoe plant was integrated to an extent into the new religion. In some way that is not presently understood, this may have led to the widespread custom of kissing under the mistletoe plant during the Christmas season. The earliest documented case of kissing under the mistletoe dates from 16th century England, a custom that was apparently very popular at that time