Mistletoe has long been associated with peace. However, no one completely agrees on the origin of kissing under the mistletoe.
Ancient Romans believed that mistletoe had the magic power of peace. If opposing soldiers met under a mistletoe-covered tree, they would temporarily put down their weapons and declare peace for the day. In later times, in both England and Scandinavia, it was customary to hang mistletoe over doorways. Anyone who passed under supposedly came in peace and was greeted in a friendly manner, perhaps with a kiss. This probably accounts the the current custom of kissing under the mistletoe.
There are many legends about mistletoe. One Scandinavian legend states that Loki, the god of destruction, killed Baldur, the god of peace, by shooting him with an arrow made from mistletoe. Other gods and goddesses were saddened by Baldur’s death and asked that his life be restored, which it was. In appreciation, his mother Frigga hung up the mistletoe and promised to kiss all who passed under it. Because of this, mistletoe became the symbol of both forgiveness and love.
In Englad’s Victorian era, it became stylish to kiss under the mistletoe. Many homes used a “kissing bow” made of two loops decorated with apples, oranges, mistletoe, and other greenery.
Source: Do Fish Drink Water? by Bill McLain