For a species to survive, its members must carry out such vital functions as eating, reproducing, and responding to aggression. Evolution has therefore developed certain areas in our brain whose role is to provide a pleasurable sensation as a “reward” for carrying out these vital functions. These areas are interconnected with one another to form what is known as the “reward circuit”. From:http://thebrain.mcgill.ca/flash/d/d_03/d_03_cr/d_03_cr_que/d_03_cr_que.html