There are a number of reasons why a dog may lick his owner, including happiness or to let its owner know it is hungry. Since a dog is unable to speak in a language humans can understand, it must lick, bark and do other things to communicate.
When a dog licks, it could mean that it is simply happy to see his owner. A dog learns to lick at a young age from its mother, who uses its tongue to clean and groom the puppy. As it grows older, a puppy begins licking to groom itself and its litter mates. Licks are also used among pack members as a way to bond.