If your dog thinks it is stronger and smarter than another human or dog, it's dominant over that dog or human. It will prove that status by correcting other living things when they "cross a line" established by the dog, i.e. if you touch his food, toys, touch him the wrong way, etc. Usually the correction is a quick snap, growl, flash of the teeth, etc. If this is not stopped early between a dog and a human, the snap could get worse, and end up being a bite. or several.