Talk with your vet about the possibility of a medical cause for the aggression, particularly if you notice other symptoms. Aggression accompanied by hair loss, increased bodyweight, and lethargy may be a sign of hypothyroidism. Aggression following convulsions, when the dog appears absent, or when rapid mood changes occur may be the result of full or partial seizures. Damage to certain areas of the brain through disease such as hydrocephalus, tumors, thyroid issues or trauma can also result in aggression problems. A consultation with your veterinarian can diagnose these conditions and offer medical treatment as necessary.