What is the "evolutionary" reason behind men going bald

It makes no sense...our bodies get hairier as we grow older, yet our heads go bald? Shouldn't it be the other way around since we evolved from apes? A guy has perfectly healthy and thick hair then BAM! male pattern baldness out of no where...why does this happen? Why don't we lose hair anywhere else like we do on our head?

I am not going bald yet since I'm only 18 but all the men on my mom's side are bald so I think I'm going to be bald like an old man by the time I'm 25...

Male pattern baldness is quite common among Europeans, and it is relatively rare among Africans and Asians. The reason for it is a gene that is carried by Europeans and people of European origin (such as many Americans and Australians). This gene causes an increase in DHT (Di-hydro-testosterone) production. DHT promotes facial and body hair growth, but DHT can also shrink hair follicles and will also cause premature baldness.

All non-Africans are descended from Africans that migrated out of Africa starting about 60,000 years ago. Some of these Africans ended up in Central Asia, and later some of the Central Asians entered Europe about 40,000 years ago, during the last ice age. Europeans, therefore, could benefit from an increase in body hair because they were living in a cold climate as they entered Europe, since hair is good insulation, and helps keep the body warm. In contrast, Africans have very little body hair because they live in a hot climate and need to sweat efficiently. The same is true of many people around the world that left Africa but ended up in warm climates, e.g. Australian aborigines and many Asians. DHT levels therefore remain relatively low in people living in tropical climates, and these people have little facial hair or body hair. In contrast, it is estimated that 40 million Americans are affected by premature baldness because most Americans descended from European ancestors.