By far the greatest culprit behind male baldness is DHT. DHT (Dihydrotestosterone) is derived from the male hormone, "Androgen". Androgen in the bloodstream interacts with a certain enzyme (5-alpha reductase) and gets converted into DHT. Short version: Androgen + 5-Alpha = DHT.
DHT is the hair follicle "strangler". When there is enough of it in the bloodstream, it will bind to the hair follicles, choking out the hair's growth. Not suddenly. At first, the hairs grow thinner because of the condition of the constricted follicles. Eventually, no hair at all emerges from the follicles.