The two main factors in which men normally lose their hair: genetics and hormones.
Harmonal Cause :
the chemical imbalance of a hormone called DHT or dihydrotestosterone is a common cause of male pattern baldness. DHT is a chemical derivative of testosterone, created when the androgen gets mixed with an enzyme called 5-alpha-reductase, DHT hinders the proper growth of hair follicles. These factors contribute to the gradual shrinkage of the tiny cavities in the skin at the base of hairs, known as scalp hair follicles. Hair grows progressively shorter and finer until no new hairs grow.
Genetic :