This is a tradition that is well know around Christmastime and it got me curious as to how this tradition started. There are no definite links back to the origin of kissing under the mistletoe, but there are couple of different legends and not even the legends are not quite the same. One of my favorites is the Norse legend and here is one version.
Baldur was the son of the king Norse gods Odin and his wife Frigg. Baldur started having nightmares of his own demise and so Frigg, being the dedicated mother that she was, decided to take action to protect her son. Frigg set out to ask everything from the four elements of fire, water, air and earth for a promise that no harm came to her son. She asked every being and plant to not harm her son. Frigg had asked everything except she had overlooked mistletoe.
Loki an evil spirit realized that Frigg had overlooked mistletoe and made an arrow out of the mistletoe wood. Loki then went to Baldur's blind brother, Hoder and guided his hand to to shoot the mistletoe arrow that killed Baldur.
It is said that Frigg's tears and grief turned the mistletoe plants berries white and brought Balder back to life. Frigg was so grateful that she declared that the mistletoe a symbol of love and promising and that no harm should come to those under the mistletoe only a kiss as a token of love.