Before you pluck off the berries and toss the whole thing away, ask yourself what you've been doing. Have you been squandering the hidden potential of mistletoe on coercing kisses out of unsuspecting victims? Didn't your sorcerous apprenticeship teach you that it was useful for so much more? Well, that's what the druids said, anyway. The Celts and the Norse, too. Of course, it was sacred to them. As a tea it was considered an antidote for poisons (despite the berries themselves being poisonous) and a cure for sterility and barrenness. So you see, kissing someone under mistletoe is actually a symbolic attempt at invoking its hidden qualities.
Just bear with me. Knowing these sorts of things, or at least pondering your way through to them, can be just the sort of exercise your mind needs when you're writing a story or building the elements of a world. It may apply to only a single system of magic or it may end up being the framework for a score of them. It's your writing so that's for you to work out. Allons-y!
In the French comic strip Asterix le Gaulois, the tribal wizard, Obelix, uses his golden serpe to collect mistletoe for potions that confer superhuman strength like Popeye’s spinach. Strength equals virility and from there it's a short hop to fertility. To the Anglo-Saxons and the Germans, mistletoe was a product of the oak and thus inherently fertile. In cultures across pre-Christian Europe, mistletoe was seen as a representation of divine male essence (and thus romance, fertility and vitality), possibly due to a resemblance between the berries and semen.
Botanically unique, mistletoe is not only a highly evolved flowering plant, but also parasitic (capable of using over 200 species of trees and shrubs as host). To the magician, this speaks to unusual properties, but curing barrenness and acting as an aphrodisiac? There’s no known chemical that has a specific, predictable aphrodisiac effect on humans. Sorry, gang, but human physiology is far too complicated and individually reactive for that. Alcohol, for example, isn’t an aphrodisiac. It merely lowers inhibitions, some of which may be sexual. Most drugs don’t stimulate arousal at all, rather they create fatigue and a sense of stimulation as that fades. Playing with brain chemicals does amazing things, but if mistletoe is lowering inhibitions they do seem to be the ones focused on kissing.
Among the Germans, decorating trees for religious effect goes back to pagan practices, the bringing of trees into the home for Winter solstice long-preceding Christianity. Teutonic religions borrowed heavily from the Celts, with whom trees were very popular, so the notion spread rather readily. In the 8th century, St. Boniface dedicated the fir tree to Christ, supplanting Thor and Odin’s sacred oak. By the 13th century, the mistletoe’s subtle powers carried it on to become the unexpected killer of the Asgardian god Baldur in the form of a projectile, either an arrow or a dart (in the Prose Eddas). With Frigg, his mother and Odin's wife, weeping over him, her tears became the white berries as they fell upon the plant and restored her son to life. Her vow that mistletoe would cause no further harm was extended to hold that those under its sway would, if enemies, set aside their weapons and embrace. Those without enmity took it even further and...kissed. After each encounter, a berry was plucked until all were gone.
An alternate version of Baldur's death, the Gesta Danorum version, casts the deadly weapon as a sword named Mistilteinn, the Old Norse for "mistletoe".
Magic arrows? Magic swords? Now we’re talking about some action. Mistilteinn makes its way into several Norse legends that way, but the plant itself never really fades completely from people’s minds. In later times, ol’ St. Nick was supposed to have had his first kiss with the future Mrs. under the mistletoe, so that probably has something to do with it all.
Magic responds to emotion and need. Apparently, collectively, we're even bigger fans of kissing than we are of magic swords. Somewhere along the way, we remembered that Love is some pretty potent magic (that version of Baldur's death with the magic sword had him killed by his brother in a rivalry over Love). That's cool in its own way. Even moreso because it's giving me still more story ideas. That's hot stuff. That means it's writing time.