We have genetic inheritance as the major influence determining our hair type as we do with skin type. It is also generally considered that diet, illness and treatment, plus seasonal weather changes are contributing factors, as with our skin.
The average head has 100,000 to 150,000 hairs and sheds between 50 to 100 of these for replacement each day. Volume depends on the number of hairs on the head, not necessarily the thickness of each strand. People with thin hair have about 90,000 strands on their heads while those with thick hair have nearer 150,000. Hair loss (or thinning) is usually attributed to the natural age progression with cell regeneration slowing down, along with the factors previously mentioned.