Do dogs just get crazier as they age? We have two dogs that have been together for 9 years and suddenly one is trying to kill the other. It's been a real nightmare and now we have a muzzle on the larger dog; she is the attacker. The vet can't find anything wrong with her so I guess it is just aging? It's really hard and very traumatizing to go through. A dog fight is violent and disturbing.