Why Dogs Bark at People

Image by Lee Coursey via Flickr
I have a German Shepherd dog that is 3 years old. Since I have a dog I am interested in dogs and sometimes I find it interesting to understand them better. For instance why they bark at all? My dog is well mannered and it is trained not to bark at people. But anyway if dogs are not trained they bark at them. My dog barks when it notices danger or when he sees something that he strongly dislikes. For instance when he was young I punished him once hitting him with the turtles metal wall decor in our living room because it was at my reach at the moment. He memorized that and now when he sees other wall decor he wants to bark at it (I trained him not to, it took me a lot of efforts). But he is not barking turtles or other metal objects. So why dogs bark at people? I think they do that because not long time ago people were the major thread to each other. I think this is genetically imprinted in dog’s minds and this surely is an evidence for our violent past.