Are any of you familiar with Norse mythology?
Well that's where the tradition of kissing under the mistletoe stems from.
The story goes like this
An arrow made of mistletoe killed Balder (One of Thor's Brothers)
source
In a version of this story his mother Frigg
source
brought her son back to life with her tears which turned the red mistletoe berries from red to white.
Frigg was so overwhelmed with Joy after her son came back to life, she blessed the mistletoe and promised a kiss to anyone that would walk underneath it.
Next time you kiss someone underneath a mistletoe, just remember that you are actually honouring Balder the Norse god :)
If you learn't something today and enjoyed my Today I learn't then please give me a follow 
for more interesting facts
Source:
http://www.sciforums.com/threads/kiss-under-the-mistletoe-may-be-the-christmas-tradition-but-you-actually-honor-balder.153835/
https://en.wikipedia.org/wiki/Baldr
http://norse-mythology.org/gods-and-creatures/the-aesir-gods-and-goddesses/frigg/