According to a custom of Christmas cheer, any two people
who meet under a hanging of mistletoe are obliged to
kiss. The origin of this custom may be related to the
story of Baldur coming back to life because of his
mother Frigga, the goddess of love who removed the
mistletoe's poison with her tears. When Baldur came back
to life she kissed everyone who passed underneath the
mistletoe out of happiness and gratitude and thus
started the custom.
Roman sources also mention mistletoe was used by the Celts in some sort of fertility rite or charm making the practice of kissing under it even more intriguing. Hence why mistletoe is used at Christmas, a time of love and joy.