There are many reasons, but the most common is due to genetics. The condition of Androgenetic Alopecia is where the hair has an allergy to the hormone chemical DHT, which wraps around the pailla of the hair follicle and cuts off its nourishment. Over time, the hair becomes smaller until it finally falls out. This is the cause of over 90% of male pattern baldness.
Age, stress, chemical and toxic exposure, hormones, accidents, and certain ailments also can cause hair loss.