Dogs can be aggressive because they are in pain, can't think straight, or have a brain disease or tumour. Talk to your vet before you try to fix the problem yourself.
Fearful dogs can become aggressive. Rescue dogs may have been abused or neglected. Get advice from a vet and an instructor; socialise and train. Approach unknown dogs with caution and avoid instilling fear.
When a dog is possessive of something, they may growl or bite if approached. It can be directed at people, pets, or valuables. Dogs and objects' aggression levels depend on their value to the dog.
Dogs may act aggressively as a way to show who is in charge, but this is often misunderstood. Dominant behaviour is not a trait; it is a response to a situation.
Redirected aggression is common in dogs who can't get to something. It can lead to lash outs at the owner or other pets. Before assuming the cause, rule out health issues and seek professional assistance if necessary.