Why do men go bald from the top of their heads?Why doesn't it spread to the sides?

Why do we lose hair? The simplest answer we know is the genetic/hereditary coding of a human body which triggers hair fall or hair growth. Although, Hair fall or Hair loss is inevitable, the degree of extent up to which it occurs is a thing of concern. For some, it is barely noticeable but for some, it is distressing and agonizing.

Since I have to keep this post concise, I am not going to discuss the types of baldness in males.

Androgenic alopecia- It is a genetic condition that can affect both men and women. Men with this condition, called male pattern baldness, can begin suffering hair loss as early as their teens or early 20s. It's characterized by a receding hairline and gradual disappearance of hair from the crown and frontal scalp.

To put it simply, some hair follicles are sensitive to DHT (dihydrotestosterone) hormone while some are not. This is one of the major reason for premature baldness in males. Hair follicles present on the top of the head have those receptors with which DHT combines and produce an undesirable effect( hair loss).

During puberty male sex hormone Testosterone is naturally converted to a stronger and more powerful hormone called DHT (dihydrotestosterone) in the presence of 5-alpha reductase which is an essential androgenic sex steroid hormone for expediting secondary sexual characters in males. For instance, DHT promotes muscle mass and bone density, deepens the voice, burgeons facial hair.

DHT is analogous to friction-'a necessary evil'.

DHT stimulates androgenic hair follicles, not the regular head hair. Facial hair, pubic hair, axillary(armpit) hair all fall under the category of DHT stimulated androgenic hair follicles.

DHT exerts a positive proliferating effect on androgenic hair follicles. However, it exerts a negative toxic impact on the non-androgenic head hair. It kills the head hair follicles by swelling the skin where hair follicles are present which results in the blockage of the necessary and vital nutrients needed by the follicle to grow hair. Consequently, hair growth stages are severely affected. The anagen growth phase of the hair follicle keeps on shortening and thus, the hair will keep shortening and thinning too until no hair can be popped out from the epidermis layer of the skin.

Why doesn't it spread to the sides?

These DHT receptors on head hair are not evenly distributed, They may be more congregated on one region of the head than other. Thus, the more DHT  receptors present on a region, the more sensitive you are to DHT on this region, the more effect DHT exerts on this region (hair loss).

The temples and the crown are the most sensitive to DHT. This is why they are the first to be affected. The hair on the sides of the head is the least sensitive. As a result, minimal impact occurs by DHT.