Sometimes it appears that a dog licks excessively. Dogs will lick for a number of reasons.
Licking is primarily a submissive social signal allowing dogs to show deference to dominant "pack members". Puppies will lick to solicit food from their mothers when they are being weaned and continue to lick to show that they are hungry. Other dogs lick as a sign of affection or because they like the taste of your skin.
Licking is usually harmless however if carried to an extreme can be a problem and a sign of stress, boredom or alleries. Obsessive dogs will sometimes lick their fur off injuring their skin requiring a veterinarian's intervention.
If your dog is licking in excess, a vet visit may be required.