Why do people kiss under the >mistletoe?

The Answer:
The custom of kissing under the mistletoe originates in Norse mythology. According to the Scandinavians, the handsome and gracious god Balder had a premonition about his murder. To prevent the death, Frigg, his mother, made every living thing promise not to kill her son. Her only omission was the insignificant mistletoe.

Loki, the evil god, discovered Frigg's oversight, and sought to exploit it. Appearing as an uninvited guest at a banquet in Valhalla, Loki watched as the other gods shot arrows at Balder for fun, marveling at how none of the arrows pierced the seemingly invulnerable god. Loki, always seeking to harm, also shot an arrow at Balder, but his arrow was made of mistletoe, so it killed him.

Though clearly innocent by modern standards, the other gods were angry at the mistletoe for killing their favorite god. As retribution, they allowed Frigg to do what she wanted to the plant. Rather than hurt it, Frigg, the goddess of love, decided to make the mistletoe a symbol of affection, asking that anyone standing under it be given a kiss of love and forgiveness.

(Source: THE STRAIGHT DOPE column by Cecil Adams)