"Imagine a set of people all living in the same building. Half of them think it is a hotel, the other half think it is a prison. Those who think it a hotel might regard it as quite intolerable, and those who thought it a prison might decide that it was really surprisingly comfortable. So that what seems the ugly doctrine is one that comforts and strengthens you in the end. The people who try to hold an optimistic view of this world would become pessimists: the people who hold a pretty stern view of it become optimistic."
--C.S. Lewis, God in the Dock

I can't decide if I like this quote, I may need to read it in context as I haven't read this Lewis book.

The title of the book makes me think it is a work of theodicy which suggests I should read him as making a defense of the Christian doctrine of original sin and the Fall. So I would read the claim this way: That what ostensibly makes a Christian a pessimist w/ a "stern view" of the world ultimately promotes optimism, or (more accurately) hope.

But I think the meaning of the quote turns on the phrase "view of this world" and I can't help reading it as a more pedestrian claim of an irony about pessimism and optimism generally. Specifically that if you have low expectations you can more easily be pleasantly surprised by positive results.