What Can Cause Slow Hair Growth?

I am a healthy seventeen yearold who still hasn't had any medical problems, or a bad medical history. My concern though is my slow hair growth. The last time i recieved a haircut was a year ago and since that date my hair has only grown 2-3 inches. I talked to my doctor about it and he said "it must just be how my body works", I believe other wise though. Can someone help please !