There are many reasons for hair loss. Genetic factors such as male pattern baldness to nutritional deficiencies can all play apart of hair loss.
When I was in my teens, there was a short period of time that I woke up each morning with huge clumps of hair, my hair, on my pillow. It started suddenly and just as suddenly stopped. It could have been caused by stress or hormones or even being sick. Thank goodness my mother did not panic and start buying all kinds of hair loss products. No doubt if she did and I used one and my body stopped shedding hair on its own, the product would get the credit instead of just my body going back to being right on its own again.
Before you start investing in hair loss products, find out if your body is out of whack in some way. Eliminate the natural causes of hair loss before you start using any kind of product.