“Of all evils, pain only is sterilised or disinfected evil. Intellectual evil, or error, may recur because the cause of the first error (such as fatigue or bad handwriting) continues to operate; but quite apart from that, error in its own right breeds error — if the first step in an argument is wrong, everything that follows will be wrong. Sin may recur because the original temptation continues; but quite apart form that, sin of its very anture breeds si nby strengthening sinful habit and weakening the conscience. Now pain, like the other evils, may of course recur because the cause of the first pain (disease, or an enemy) is still operative: but pain has no tendency, in its own right, to proliferate. When it is over, it is over, and the natural sequel is joy. This distinction may be put the other way round. After an error you need not only to remove the causes (the fatigue or bad handwriting) but also to correct the error itself: after a sin you must not only, if possible, remove the temptation, you must also go back and repent the sin itself. In each case an ‘undoing’ is required. Pain requires no such undoing. You may need to heal the disease which caused it, but the pain, once over, is sterile — whereas every uncorrected error and unrepented sin is, in its own right, a fountain of fresh error and fresh sin flowing on to the end of time. Again, when I err, my error infects everyone who believes me. When I sin publicly, every spectator either condones it, thus sharing  guilt, or condemns it with imminent danger to his charity and humility. But suffering naturally produces in the spectators (unless they are unusually depraved) no bad effect, but a good one — pity. Thus that evil which God chiefly uses to produce the ‘complex good’ is most markedly disinfected, or deprived of that proliferous tendency which is the worst characteristic of evil in general.”
- C.S. Lewis, ‘Chapter 7: Human Pain, Continued‘, The Problem of Pain