Puppies are descendants of wolves. A wolf has a pack leader called an Alpha and Omega. To show submission, they lick the leaders mouths. So for your puppy to lick you like that, they know that you are the leader.
Your puppy licks you all of the time because he loves you and is showing affection. The way your skin feels on your puppy's tongue could be another reason that he is licking you all of the time. You could just smell so good that your puppy wants to lick you.