Why does my dog try to attack me?

our family keeps him on a very long runner because of the neighborhood traffic and generally he is a good smart dog hes a German Shepard and every time i go out to see him he is fine when i walk within his reach but when i go to pet him he snarls his lips and barks like hes crazy. i have had one instance where he broke his chain and successfully and kindly let me put a leash on him pet him on the head and back and bring him back home. i just dont understand i dont have a fear of him until after he freaks out so why does he?