Wrinkling Of The Muzzle
Unfortunately, not all dog behaviors mean that they’re happy and want to be friendly. When a dog wrinkles their muzzle, that is most likely a sign of aggression. If they go a step further to show their teeth and even snarl, you should take that as a warning to stop whatever you’re doing that is getting them worked up or remove yourself from the situation.
Actions like these mean that the dog is ready to attack at any time.
You should feel honored when a dog exposes their belly to you.