Previously calm, now suddenly wants to bite, growls, or won't let anyone near — many people think it's a training issue, but in reality it can be a sign of pain, neurological disorder, or anxiety disorder. When a dog is in pain somewhere in the body, it becomes aggressive as a form of self-defense. In this article, you'll learn which behaviors are red flags, when you need a behavioral vet, and how to create a safe environment and gradually get your dog back to a normal mood. Behavior change = health change — never ignore this rule