From Marriage Matters: Extraordinary Change Through Ordinary Moments by Winston T. Smith:

When you are not feeling particularly friendly but know you ought to be, the best thing you can do 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. Often the only way to get a quality in reality is to start behaving as if you had it already.