Whatever men expect they soon come to think they have a right to: the sense of disappointment can, with very little skill on our part, be turned into a sense of injury. It is after men have given in to the irremediable, after they have despaired of relief and ceased to think even a half-hour ahead, that the dangers of humbled and gentle weariness begin.
 C. S. Lewis, The Screwtape Letters (1942). copy citation

Context

“The paradoxical thing is that moderate fatigue is a better soil for peevishness than absolute exhaustion. This depends partly on physical causes, but partly on something else. It is not fatigue simply as such that produces the anger, but unexpected demands on a man already tired. Whatever men expect they soon come to think they have a right to: the sense of disappointment can, with very little skill on our part, be turned into a sense of injury. It is after men have given in to the irremediable, after they have despaired of relief and ceased to think even a half-hour ahead, that the dangers of humbled and gentle weariness begin. To produce the best results from the patient's fatigue, therefore, you must feed him with false hopes. Put into his mind plausible reasons for believing that the air-raid will not be repeated. Keep him comforting himself with the thought of how much he will enjoy his bed next night.” source