Most animals attack out of fear, you need to seek proper training asap before he/she hurts someone and they take the dog away and put it down.
He hasn't been trained properly. With watching dog training shows, and Dog Whisperer, I imagine this is not the only thing he does that he should not be doing? Such as barking at the door or not obeying you, perhaps not eating when he should be or tearing into stuff that he shouldn't be in? If you can't afford to get him training lessons, the shows they have on TV are quite helpful, and with persistence, will pay off. Another thing I might suggest is to get him neutered.. Least that's whats recommended on the shows.
You need to have him trained properly, and then you need to put him through obedience school too. If this isn't done soon he will most likely maul someone, and then will be put down because of it. Hope this helps, good luck to you.
Maybe your dog has been around an unsafe environment or neighbors have been cruel to him or he just is protective of you