Why my german shepherd is very aggressive to me ?

My Puppy Is Calm And When he Bites he Bites Normaly as any puppy biting But When i play Such games like fetch a ball or sumthing he becomes very aggresive and Bites it Agressively And When i try To Take it he Bites Me Aggressively So i wanna know any solutions Which Can Make Him Be Normal When Fetching And Not Aggressive To Me :O