There seems to be this notion of self-denial being not only the means to an end, but the end itself. It is as if the desire of a reward promised is a bad thing. In some cases, it is. But in the case of a lover desiring marriage or a Christian desiring heaven, the desire is a good and healthy thing to have. There are natural rewards and unnatural ones. If a man marries a woman for the reward of her money, that is not natural. Money is not a natural reward of love. Unnatural rewards are not to be sought after by any means.
Lewis talks about a third type of reward. This is the reward that we do not fully understand until we have it. There is not really a desire of this reward as there is a desire for a lover to marry. Lewis relates this to a schoolboy learning Greek. A schoolboy cannot fully understand what it is like to love and enjoy Greek poetry. He simply must work for marks and to avoid punishment first. It is not until he approaches the reward that he can fully understand and desire it. And, the closer he gets to the reward, the more he desires it. Lewis relates the situation of the schoolboy to our situation in our strivings for Heaven and nearness to God. We cannot fully understand heaven or know why exactly it is that we desire it. We simply need to trudge on and hope that we will indeed receive a reward that is more than worth our strivings. And, the longer we go on and the closer we get to God, the more we will desire Him.
Lewis claims next that although we ought to be desiring future glory, since we cannot grasp what it is we are desiring we are far too content. Because we cannot grasp the glories of heaven, we are completely satisfied "playing with mudpies." Future glory should be our main focus and our only desire but as a result of the fall, our vision is cloudy and we are perfectly content with it as such.