Mouth licking

tesuhoha

I was going to say maybe the dog is attracted by the smell of her dentures - before I read the other posts.

krlyr

Mouth/face licking is an appeasement signal usually, and quite common for pups. He should likely grow out of it, sounds like the old girly is just quite a tolerant dog - others may tell him off (and he'd learn not to do it). You might perhaps want to consider discouraging him (not punishing him, but calling him away before/when he does it) if it's a bit excessive, just so he doesn't develop it as a habit (which could get him in trouble with the wrong dog).

It could also be a sign that he's a bit anxious/uncomfortable - perhaps in general, perhaps with that particular dog or situation, so worth looking into that too.

http://dogtime.com/dog-health/dog-behavior/3472-dog-speak-face-licking-colleen-safford