What Causes Lack Of Facial Hair Growth?

sir my name is ishan gupta.i am 21 yrs old.......i ahve no problem with my health.i am tall and fit person.i ahve problem that i have only small bear on my face.my all classmates have proper bear but i havent yet......i am getting deppresed day by day by facing this type of problem..sir what i do fro getting out of this problem