Under the Misteltoe wasn't always a place to be kissed.
In Roman times, this white-berried plant was more a symbol of peace than romance.
When people warring with each other met beneath a mistletoe covered tree, they would throw down their weapons, at least temporarily.
Later on, in England and Scandinavia, the plant was hung over entryways, anyone passing under it was supposed to come in peace and would receive a friendly greeting, such as a kiss.
Todays romantic custom of kissing under the mistletoe was probably derived from this.
And part of the fun, according to tradition is that you can't refuse a kiss from anyone who catches you there!