My Bruno used to do this.
He also never did it to me...but from excitement would love to hold onto someones hand. Never hurt them....but a bad habit.
I told people that would come over to not pay attention to him for a few minutes and to tuck their hands under their armpits or into their pockets. Not hands...no mouthing.
He did get much better in time...but in all honesty we never did break the habit completly. When a favourite person came over... he would try it.
The day we had to PTS at the age of 5 because of HD....the last thing he did was die with my hand in his mouth
Gina