You'd sometimes be forgiven for thinking that a friendly smile costs money. Some people just won't part with one for love or money (well, I've never tried money, so maybe this statement is a bit too sweeping).
Recently I went out to lunch at a local restaurant with a small group of people. The weather was fine and the tables on the terrace were all taken. Inside the restaurant quite a few tables were also occupied so we decided to move to the upstairs area. This didn't suit the waitress who was apparently not enthusiastic about having to come upstairs to serve. She looked at us as if we'd crawled in on our under-bellies and was extremely slow in taking our order. By the time she came back with the food the place had filled up and every table upstairs was occupied. So her disgruntlement was all for nothing as we were not the only customers she had to serve in that area. What she had done, however, was ensure that I would not use that restaurant again. If management are pleased to employ someone so unsuitable then they do not merit my custom. And, of course, human nature being what it is, I have related this incident to all my friends and acquaintances. Maybe she was having a bad hair day, but quite frankly, how would I know that? I took her at (unsmiling) face value. I'm not about to psycho-analyze every waitress I see - let's face it, I'd be locked up if I leaned across the table and tried a bit of Freudian therapy on her.
Various studies have revealed that smiling, even when you don't feel like it, can reduce stress regardless of your actual mood. If you act cheerfully, you will become cheerful. Interesting, isn't it? I expect the opposite is true, too. If you go around with a mournful face and expect rain, then you are going to feel grumpy or sad and you may even get a downpour.