What Causes Reduction In Hair Density?

Hi Doctor, Problem : Less hair density I used to have thick hair(or atleast i thought i had) until last year before i shaved my head. After that i began to notice the lessening density everyday when it started growing back. It was evident that density has become very less, say from 100 to 40. I don t believe that shaving my head was the reason for this dramatic decrease. So what might have happened? And also, could massaging with natural oils help it grow back gradually? Since, I don t want to go for medications.