Yawning is a form of dog body language. When you see your dog yawning, there's a good chance he's trying to communicate with you.
What It Is
Yawning in dogs looks just like it does in humans - wide open jaw accompanied by a big, deep breath. While with people we usually associate yawning with fatigue, when dogs yawn, it is a form of dog communication.
What It Means
Yawning is a type of appeasement gesture, something also referred to as a calming signal. Dogs yawn to deflect a threat. If a person or another animal approaches a dog, that dog may avert his gaze and yawn. It's a dog's way of saying that he feels threatened or anxious, but that he is not going to attack. Dogs use this type of body language to avoid conflict.
Keep an eye on your dog, and you may catch him yawning. It could happen if two children are fighting close to where the dog is lying down, if a child hugs him, when someone scolds him, or in a variety of other stress-inducing situations. Being aware of what causes anxiety in your dog, can help you prevent him from being exposed to those situations. Some people even yawn themselves to "talk" back to their dogs. Many notices that yawning when their dog is stressed helps to calm the dog down.