I think one may be quite rid of the old haunting suspicion—which raises its head in every temptation—that there is something else than God—some other country . . . into which He forbids us to trespass—some kind of delight which He “doesn’t appreciate” or just chooses to forbid, but which would be real delight if only we were allowed to get it.
The thing just isn’t there. Whatever we desire is either what God is trying to give us as quickly as He can, or else a false picture of what He is trying to give us—a false picture which would not attract us for a moment if we saw the real thing. Therefore God does really in a sense contain evil—i.e., contains what is the real motive power behind all our evil desires. He knows what we want, even in our vilest acts: He is longing to give it to us. He is not looking on from the outside at some new “taste” or “separate desire of our own.” Only because He has laid up real goods for us to desire are we able to go wrong by snatching at them in greedy, misdirected ways. The truth is that evil is not a real thing at all, like God. It is simply good spoiled. That is why I say there can be good without evil, but no evil without good. You know what the biologists mean by a parasite—an animal that lives on another animal. Evil is a parasite. It is there only because good is there for it to spoil and confuse.
Thus you may well feel that God understands our temptations—understands them a great deal more than we do. But don’t forget Macdonald again—“Only God understands evil and hates it.” Only the dog’s master knows how useless it is to try to get on with the lead knotted round the lamp-post. This is why we must be prepared to find God implacably and immovably forbidding what may seem to us very small and trivial things. But He knows whether they are really small and trivial. How small some of the things that doctors forbid would seem to an ignoramus.
From The Collected Letters of C.S. Lewis, Volume II