To deal with intimidating
Certain dogs breeds (Pit Bulls, Dobermans, Rottweilers, St.Bernards) also have the temperament to be aggressive while other dogs may have an association to the unfamiliar that triggers them to react in an aggressive manner (even though they aren’t necessarily in a situation where their survival is indeed threatened).
When this occurs, and your dog realizes that you, and not him or her, are at the top of the order, you will find that your dog experience to be greatly enhanced.
As a result, refashioning this pack order is crucial.
You must make sure that your dog is at the bottom of the pack and that your dog realizes that you are at the alpha leader instead. For example, you have to be a more firm with a dog that does not consider itself at the bottom of the pack.
This is why approaching an unknown dog is always discouraged.
Another reason behind aggressive behavior could be a medical condition.