why do people hate my dog?

I went to the dog park yesterday with my, little over a year old pup, American "Pit Bul" Terrier ass soon as i gbot out of the car and walked to the park peopl elooked at me and they ALL and i mean ALL left why? what can we do to educate the uneducated, my dog is the nicest "Pit Bull" he graduated from puppy-beginner, advanced and intermidiate. do we need a sign that say i will not bite? or what. so many people judge my pup why? serious pit bull owners only please..