Why is my puppy so aggressive?

NEVER EVER smack your dog or wrap your hand around her muzzle. This only gets them angrier and gives them a reason to fear the hand. The best thing to do is desensitize her to hands and human contact by giving her treats, but you must hold the treat so she cannot eat it all, and then hold your other hand close to her, relaxed. Do this five times a day or as often as you can, and then work your way towards resting your hand on his back for a second to a couple and then petting him and touching her head.