They use their tongue to express emotion

Dogs use their tongues to show you affection and this is why they like to lick your face when you come back home. 

“Dogs use their tongues to lick other dogs’ faces during times of happiness and excitement,” Reiter adds.

Nevertheless, try to limit the moments when your dog can lick your face because their tongues contain certain bacteria that can affect your health or can trigger allergies. “There is some research that bacteria causing periodontal disease can transfer from dogs to humans,” Reiter says.