My mother has little one at home that will jump onto the back of the sofa in order to reach my mother's face. If my mother blocks her, than she will move on to her arm or legs. No one knows for sure why dogs lick, but there are plenty of theories.
The first thing a mother will do when a puppy is born is to lick it. This is done to stimulate breathing and to clean the baby of the blood and fluids associated with birth. It is important to keep predators away from the babies; so that the predators don't smell the blood.
It Just Feels Good
Licking is hypothesized to release endorphins in the dog. This is similar to why human enjoy exercising. These endorphins give dogs feelings of comfort, happiness and safety. Perhaps it is similar to cats that purr when they are happy.
Sign of Affection
Licking is simply a sign of affection. Dogs show that they love their owners over other people. I have a dog that loves travelling in the car. She would actually get depressed when she hasn't had a ride in a while. Once, when she got out of the car after a ride, she was SO happy that she went and gave the tire a lick.
Maybe dogs simply enjoy the taste of skin. Humans are always sweating and sweat is salty. Dogs can have a craving for salt similar to humans.
Sign of Submission
A submissive dog will lick a dominant dog. This shows who is boss. It can also annoy the other dog as much as the human owners. One little Yorkie I knew used to love to lick the muzzle of an old male dog that she lived with. It never failed that the older dog would complain and would complain with a noise that was between a bark and a cry. The young Yorkie loved to make the older dog yelp like this. You could see that it was almost a game between the two of them.
Some people find licking an adorable behaviour, but most people find it to be undesirable and unacceptable. To stop licking, you must train your dog, just like with any other unwanted behaviour. One option is to walk out of the room whenever your dog licks. Your dog will eventually associate you leaving with licking and will then stop licking. Do not punish a dog too much for licking. Remember, now you know the answer to the question "Why do dogs lick you?"