What could cause my one indoor dog to lash out at the other dog? They’ve always been friends?
Dogs can have sudden changes in temperament towards other dogs for a variety of reasons. Some of the most common reasons for sudden aggression are illness, pain, fear or a desire for territory, protection, possession or dominance. Even if two dogs normally get along, something may be different in a situation that causes one dog to suddenly become aggressive towards the other.
Establishing a sense of territory and possession could be the most likely cause for one indoor dog to lash out at another. If a dog goes for another dog’s toy, food, or towards a favorite spot like a bed, the dog that normally claims those could lash out to show possession and territory.
A dog may also lash out due to fear. If a dog feels threatened , ganged up on or backed into a corner, primal instincts may kick in and cause the dog to lash out. This is a typical fight versus flight reaction, and many dogs will choose to fight.