Why does my puppy like to lick my older dog's mouth (inside!)?

Question

I have two female stray dogs, one is 5 yo, and the other one (the puppy) is almost 2 yo, and my puppy likes to lick my older dog's mouth.

And when I say mouth, I mean INSIDE the mouth, like the teeth and tongue.

We never teached or encouraged her to do this, we always tell her to stop doing this and try to push her away, but she never stops. The older dog growls at the puppy because of this behavior, and sometimes bites her snout when it's inside the mouth (not to hurt, just as a warning), but she never stops.

Why does she do this? And how do I make her stop it, since it's clearly annoying for the older dog?

FYI they are not mother and daughter, and only the older dog is neutered.

Answer 1

When we domesticated dogs from wolves, dogs ended up in many ways more like wolf puppies than adult wolves. One of those ways is their impulse to lick people or other dogs on the face. Wolf puppies do this when the adults return from the hunt to encourage them to regurgitate food for them to eat.

Of course it's not really usual for a dog to go as far as licking inside another dog's mouth while doing this, but that seems to be your dog just going a bit overboard in its instinct to lick.