A lot of people think that dogs lick because they love you. While that's a great thought, and I'm sure your dog adores you like none other, that's not necessarily the case all of the time. Dogs can show affection through wagging their tails, run up to you when you come home and so on and so forth.
According to Dr. Terrence Ferguson (Vet at Critter Fixer Veterinary Hospital), dogs lick people to show affection, but they also lick to relax themselves. Not to mention, dogs lick because our skin is salty, and they love the taste.
Licking can also occur due to the "wild dog" culture, show that they're hungry or are bored.