Answer:
Um, 1st the body increases the blood flow in the penis. (Don't remember how tho. I think it was related to the increase of adrenaline a body produces? But that was maybe about blushing etc) and 2nd, might be because the nearby blood vessels have blockages, or if we go the other way, the person has suffered from a psychological trauma. Also another answer for the 2nd is that the nearby nerves are damaged or that the man has had his spine damaged therefore being paralysed.
