Far too many dogs are allowed to get away with aggression and with the positive reward of the stranger going away, it becomes a self-perpetuating behavior. The dog soon learns that this coping mechanism actually works. The positive reward of being removed from the situation should be stopped. Dominant dogs determine how a pack is to react to an approaching stranger. If the alpha dog says everything is OK with this individual, then it is not up to a subordinate member to start a fight with them.
Alpha dogs determine who to fight and when to fight.
Any form of unwanted aggression is not tolerated by the alpha dog. This is how the worst cases of fear aggression are controlled. It relies on two things - that the alpha member is present and that the alpha leader is in control of subordinates. If you are not the alpha leader of the pack and your dog is, then you will have no way to control your dog's reaction. If you don't know whether you are boss or not, you need to take the dominance test .