I have never understood why dogs gravitate toward their owner’s mouth. They say the eyes are the windows to the soul. Maybe to a dog the mouth is the window to what you ate last.
salt.
Good hypothesis.
It is a submission thing. Watch any pack of dogs, wolves, or coyotes, and the submissives will lick the alphas teeth and lips. Really common, especially in younger dogs. And since we are the alphas they show us the submission.