Since mistletoe remains green throughout the year, many ancient peoples have ascribed these plants with magical healing powers and with fertility, and some cultures viewed it as an aphrodisiac due to the suggestive arrangement of its berries. Apparently, the association between mistletoe and fertility made it a traditional addition ancient Greek myths, where the custom of kissing under mistletoe may have started. This practice was later extended to wedding ceremonies.
Victorian England seems to have adopted this tradition, too. For example, if a girl refused a kiss whilst standing under mistletoe, it was said that she wouldn't receive any marriage proposals during the following year. Worse, it seems that many people would avoid her since they believed she would probably end up an old maid -- as if this was such a bad thing!
Interestingly, there is a proper etiquette for kissing under the mistletoe: first, the man can only kiss a woman or girl on the cheek and second, when he does so, he removes one berry from the mistletoe sprig. After all the berries are gone, the kissing ends, too.
Viscum album Illustration: from Charting Nature's Vintage Flower Collection