Andrea Rosebrock 
Dog owners must be aware of aggressive behaviors exhibited by their dog. A biting dog is a serious liability, and responsible owners must be proactive in dealing with the source of the aggression in order to treat the behavior.
Some dogs bite as a defense mechanism, and are actually responding to underlying fears. Identifying the fear-provoking stimulus and retraining the dog to overcome his fears is critical in treating the problem. Corrections, especially harsh, physical ones, will only serve to intensify the fears and cause further behavioral problems.
However, aggression is not always associated with fear. Overly dominant dogs may aggressively respond to the presence of new dogs. Territoriality may also underlie aggression. Dogs may act aggressive to dogs of the same sex, but not to those of the opposite. Some dogs are even taught, either consciously or subconsciously to exhibit aggression. Many of these dogs can be reconditioned.
Aggressive acts can also be the first sign of underlying and treatable medical problems. Dogs who suddenly start to act aggressively could be responding to pain associated with orthopedic abnormalities, gastro-intestinal discomfort or serious illnesses, such as cancer or thyroid imbalances. Diagnosis and treatment of physical illness is necessary to resolve associated behavior problems.
If your dog bares his teeth or snaps at you, take the action seriously, before the behavior escalates to more serious threats. Seek professional help immediately. Ask your veterinarian to recommend someone qualified in dealing with behavior problems. Many trainers or behaviorists work in conjunction with veterinarians to treat aggression, occasionally using pharmacological approaches to treating aggressive tendencies. Aggression from dogs is intolerable, but often treatable.
Source – Dog.com
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