There is always a cause for aggression, and hopefully a cure for it and it is positively preventable. A dogs instinctive desire to dominate is the main reason for a dogs aggression.
They will always attempt to be the leader of the family, which to him is his pack. Aggression during training sessions must be dealt with immediately, he is challenging your authority to teach him, and if not corrected immediately, it will cause problems later.
If as a puppy, your dog growled and got what he wanted in that way, he was taught that aggression got him what he wanted, and next time the aggression will be worse.
They need to be taught when very young that aggression is not acceptable. Dogs also need interaction with people and other dogs, if they are not provided with this, they will learn to lung at, growl and bark at them.
If a dog is beaten or abused, he will learn to protect himself every time a hand is raised in his vicinity. Anything that is painful to a dog can teach him to be aggressive, and should never be used as a training tool.
All aggressive dogs will show some signs of aggression, even though they may not show all of the signs of aggression, some of these signs are:
*Laid back ears, tail tucked between the legs
*Crouching while trying to avoid eye contact
*Standing stiff and erect, this is a dog who is dominantly aggressive or territorial
*Direct aggressive stare, with ears and tail held straight up
With a fearfully aggressive dog, initiate a course of gradual socialization and exposure to people and dogs.
Gradual is the key word here, never try to push your dog into socialization.
With a territorial or dominantly aggressive dog, obedience training will teach him that you are in charge, never loose your temper, an aggressive dog will take advantage of you at this point.
Most dogs will show aggression at some point, this must be dealt with properly, always correct aggression when it is shown, every time.
