To some dogs, licking the face (or muzzle) is an appeasment behavior. Maybe your dog feels so comfortable with you that it doesn't feel the need to appease you. Some dogs do it as a stress relief to lessen their anxiety. Some dogs do it just "because" they like it.

I wouldn't worry about it - neither of my dogs really do it all that much. Penny Lane will occasionally do it to the vet techs (stress relief) and Leo will give kisses if I ask her to. My RB dog Buster would lick my face all the time. It just kind of depends on the dog.