Dogs lick people to communicate and to express their affection. The communication can include information, such as how hungry the dog is or that the dog is being submissive towards the person being licked.
In the canine world, dogs lick each other as a friendly greeting. Since dogs feel that the humans in their household are a part of their pack, this behaviour often gets passed on. Younger dogs are more likely to lick people, since they are less likely to know better. Some dogs never grow out of this type of behavior, while other dogs are never really lickers, depending on the personality of the dog.