Why do people kiss under the mistletoe at Christmas?

In ancient myth, when the son of the Norse goddess Frigga, Baldr, was killed by an arrow made of mistletoe and then brought back to life,
she blessed the mistletoe and bestowed a kiss on all who passed beneath it. In the 18th century, the legend was adopted as a promise to marry. At Christmas a lady standing under a mistletoe may not refuse a kiss. If she does, she cannot expect to marry the following year. So it is told.