Why do we have to clean hair from under the arms and the pubic area?
Removing hair from under the armpits and the pubic area is a Sunnah of the Holy Prophet (sws). The wisdom behind the custom is to attain physical purification. Humans, by their nature, want to be purified physically. That is why the Holy Prophet (sws) called the act to be an observance of human nature and instituted these acts as the Sunnah. He is reported to have said:
Five things are from among [the norms of] human nature: circumcision, shaving the pubes, cutting nails, removing hair from under the armpits and clipping the moustache. (Muslim: No. 257)