Sadly humans are often too quick to punish a dog for growling at people instead of sitting back and working out how they can get the dog over this fear. Recently I was given a link to a lovely video which explains so simply how to do the latter, quickly and effectively, without the need to 'boss' the dog and chastise it for bad manners. Here's the link so that you can see for yourselves. :o) Does Giving Treats Reward Aggression?
Good dog training and behaviour modification is all about using your brain, not your brawn. ::winks::