When a child is told, "If you do this, you shall have candy", however great the sacrifice is, it will do it, for it thinks, "I shall have candy." The words in the scriptures about the reward of good deeds in heaven were spoken in a manner suited to the evolution of that time. The promises were made as an older person makes promises to a child and says, "Do not take another person's apple; I will give you another apple, even sweeter than this. Don't take another child's doll; I will give you another doll, even better than this.'