Here is a quote from the article on cheerfulness I put a link to earlier.
Psychotherapist Tess Ridgeway agrees that choosing to be cheerful doesn’t mean walking on air. “Rather, it means you are committed to being a person who focuses on the good, looks for the best in people and picks yourself up from bad events with stoicism and determination to carry on. It isn’t flighty or dependent on good fortune. It’s a decision you make, to walk through life with good humour, humility and optimism.” I like that she describes it as a decision - not something you either have or you have not.