When you are not feeling particularly friendly but know you ought to be, the best thing you can do, very often, is to put on a friendly manner and behave as if you were a nicer person than you actually are.
 C. S. Lewis, Mere Christianity (1952). copy citation

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Author C. S. Lewis
Source Mere Christianity
Topic feeling good
Date 1952
Language English
Reference
Note Adapted from a series of BBC radio talks made between 1942 and 1944
Weblink https://www.dacc.edu/assets/pdfs/PCM/merechristianitylewis.pdf

Context

“There is a bad kind, where the pretence is there instead of the real thing; as when a man pretends he is going to help you instead of really helping you. But there is also a good kind, where the pretence leads up to the real thing. When you are not feeling particularly friendly but know you ought to be, the best thing you can do, very often, is to put on a friendly manner and behave as if you were a nicer person than you actually are. And in a few minutes, as we have all noticed, you will be really feeling friendlier than you were. Very often the only way to get a quality in reality is to start behaving as if you had it already. That is why children’s games are so important.” source