This may be sibling aggression (in which the dog is constantly
at odds with another dog in the household), fear aggression (loud noises, strangers, sudden movements, etc.), resource
guarding (in which dogs become overprotective of food, toys, and other possessions), leash reactivity (in which the dog reacts aggressively to other dogs or stimulants while on the leash), or the simple will to
dominate (in which a dog wishes to prove he / she is the alpha dog).