Why humans are relatively hairless?

• The running man hypothesis: we evolved to be two-legged so that we could throw spears and see far, but also so that we could separate our breathing from our stride. When most four-legged animals sprint, their bodies expand and contract such that their breathing is forced to follow their stride; we can decouple those two motions, which is a luxury. Furthermore, hairlessness helps us to sweat, as hair would slow down evaporative cooling.
• The aquatic ape hypothesis: Another idea holds that humans became bipedal because they wanted their heads up high for wading and fishing. Aquatic mammals tend to either have very dense hair or no hair at all (whales, dolphins, pigs, etc.). This idea is not as crazy as it sounds, and some random observations support that we evolved to be in or near wet environments. For example, you know how your fingertips get wrinkly when they're in water for a while? Well, that reaction is regulated by your nervous system, and is not a direct effect of wetness. Furthermore, those wrinkles have been demonstrated to aid your ability to grip wet rocks.
• The removable fur hypothesis: Basically, as humans learned to make clothes, we stopped needing fur. Fur is not as good as clothing, because you can remove and clean clothing. Fur, on the other hand, is always full of parasites. Consider the two hairiest parts of the body, the scalp and the crotch; both are subject to lice.