Does your dog exhibit aggressive behavior? If so, it is highly likely your dog feels vulnerable or under threat in those particular situations, and he will use his instinctive communication system to make his feelings known. These warning signals can include growling, the showing of teeth, raised hackles, barking, snapping, and biting and are a clear message that your dog does not feel safe or protected and will either attempt to avoid confrontation or take on the challenge of dealing with the "threat" himself. Often the results are traumatic and costly for both the dog and humans involved.
Dog aggression is a symptom of an underlying issue-usually fear. Dogs that have been attacked or even simply bullied by larger dogs as puppies can often exhibit aggressive behavior to other dogs later in life. If a dog has had a frightening experience with a human, he may exhibit aggression toward people as he may feel it is the only way to defend against perceived future threats.
Early warning signs: Growling and/or barking at strangers or other dogs are the earliest signs of aggression. If these early warning signs are recognized and dealt with immediately, the worst-case scenarios of the dog injuring other animals or humans can be prevented. It is important to deal with the problem at this early stage to prevent any seriously damaging situations later.