Mistletoe carries special meaning as a symbol of fertility. Since it retains its greenery all through winter (seemingly by magic as it has no roots) and blooms even through the darkest days of the season, it’s treasured as a token of eternal life.
The Celtic druids considered the plant sacred and hung it above doorways for luck. As early as the 1st century AD, they conjured the mystical powers of the plant in a religious ceremony called the ritual of oak and mistletoe to cure infertility.
Greco-Roman history reveals that Diana, goddess of the moon, wore a crown of mistletoe as an emblem of fertility and immortality (not to mention the berries appeared to resemble little moons).
One theory even says that the seeds of the plant gave rise to its reputation as a life-bearer because they’re sticky, like semen.
Since mistletoe is seen as a symbol of fertility and life, this could be a reason why we now kiss underneath it.