When a dog is confronted with a stressful situation, it has three choices: Avoidance, Appeasement or Aggression. For most dogs aggression occurs only when the first two options (running away or submitting) are not possible. Canine aggression takes several forms, each with different causes.
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Dogs usually try to intimidate opponents with a display of aggressive behaviour, but will try to avoid full-blown conflict (hence the phrase "his bark is worse than his bite").
Dogs will defend their territory against other dogs, as well as against humans, and this behaviour is more common when a dog's owner is absent when the dog believes it is solely responsible for its territory. Some dogs will also defent its owners against any perceived threat, real or otherwise.
Dogs being harassed by children may bite because they misunderstand the child's behaviour. Children should be taught at a very early age to treat all animals with respect and gentleness.
Quite often, persistent aggression in male dogs can be solved by neutering.