Why Your Dog Growls At Some People And Not Others

Knowing that dogs bark for various reasons doesn't answer the question posed in the title. When our dog barks at someone who is aggressive, it is understandable they might react to the situation vocally. When a good friend comes up to say hi with a friendly disposition and barking ensues, we may have less of an idea. Similarly, why does a dog not react at all to some people, but will growl or bark at others?