When I was a university student, I had a part time job in the Tennis Club as a tennis coach assistant. One day, I drunk with my friends all through the night until the next morning and went to work at the tennis court with a gloomy face because of the bad health condition.
When the coach saw my face, he scolded me and said to me that I should not show my gloomy face that could make my customer feel bad, because I was earning money from them. He also said that whether you are feeling good or not, that's not issue, but it is worth more to make your customer feel good that's why you should smile even if it is a forced one.
Until that day, I used to be pointed out to be "a person of moods" but I couldn't understand what that meant. I might have being showing an awful gloomy face when I was feeling bad. I didn't know that my gloomy face was giving such big bad influences on the people around me. I didn't realize that “Bad feelings or atmosphere were proliferated by my gloomy face around me”
From that day on, I try to smile (even it is a forced one). Smile makes people healthy, and seeing smiling people make me healthy too. Even there is no one around me I found that I could be healthy by smiling.
The Old saying “Good fortune and happiness will come to the home of those who smile” sounds very obvious, but it is very important to understand that it doesn't mean just that “you are smiling because you are happy”, but rather “you will be happy because you smile”.
And it is easy or obvious to think that you are smiling because you are delighted or happy. However, I believe that it is very important to recognize that “you should smile when you are having a difficulty or a hard time”.