While dogs don't think of kissing exactly the same way people do, it's generally a sign of affection -- for one reason or another, your dog is expressing appreciation. Even your reaction to kisses can change the meaning behind them, so consider all of the factors that can influence this behavior.
Lessons from Puppyhood
Newborn puppies are frequently licked by their mothers, so your dog's kisses could mean that he learned to associate the behavior with care and love. For example, mothers lick their puppies to stimulate going to the bathroom. In fact, one of a puppy's first interactions is that of his mother licking him clean after birth. Puppies also lick their mothers around the mouth as a way of asking her to regurgitate food. If he was frequently licked by his mother as a pup, he may pick up the behavior and direct it toward creatures he considers family, like you.
Dogs take natural comfort in licking, or "giving kisses," so he may be doing it for his own benefit as much as yours. When he gives you kisses, it releases endorphins, making the experience highly pleasurable for him -- if he doesn't like to lick you, then, he may habitually lick toys, furniture or other animals. In a pack, dogs bond and establish their submission by licking the pack leader, so his behavior may mean that he has strong instincts or a history of gleefully submitting in a family.
Savoring the Flavor
When your dog gives you kisses, it simply may mean that you taste good. Human skin gathers natural oils and salt from your sweat, and dogs can find that highly palatable. If he goes for the areas that collect the most, like your hands, fingers, feet and face, he just may be taking in the world of natural flavors collecting on your body.
Learning by Habit
Your dog's kissing behavior simply may mean that he's an astute learner. That is, many dogs lick their owners because it garners a favorable reaction. Just like training your dog to sit or shake for a treat, then, he learns that he'll earn your attention by planting a few wet ones on you. For this reason, one of the first steps toward breaking the kissing habit is ignoring your dog. If he stops feeling rewarded when he kisses you, he'll be retrained not to bother.