So what's the deal with kissing under a green plant hanging from your ceiling? Well a gazillion years ago mistletoe was considered to bestow life and fertility, was a protection against poison and an aphrodisiac.
It was also believed a young lady standing under a ball of mistletoe, brightly trimmed with evergreens, ribbons, and ornaments, cannot refuse to be kissed. Such a kiss could mean deep romance or lasting friendship and goodwill. If the girl remained unkissed she can expect not to marry the following year. Thus, if a couple in love exchanges a kiss under the mistletoe it is interpreted as a promise to marry as well as a prediction of happiness and long life.
Moral of the story...be picky under that mistletoe!