When Lewis first came to faith, he did not think about eternal life, but focused on enjoying God in this life. Lewis later said that the years he spent without the focus on heavenly rewards “always seem to me to have been of great value” because they taught delight in God above any prospect or reward. It would be wrong to desire from God solely what he could give you, without delighting in God himself.
Lewis never disparaged the place of heavenly rewards, but he saw that the paradox of reward might be a stumbling block for some. On the one hand, the purest faith in God believes in him for “nothing” and is not primarily interested in any benefits to follow. On the other hand, the concept that we are rewarded for what we do is taught in numerous biblical passages and presumably is a positive motivation for doing what is good.
Religion & Liberty Online
C. S. Lewis on selfishness vs. self-interest