I don't really have much to worry and should be happy most of the time. But I'm not.
Preoccupation with work performance and concern for future security is the main cause. So work ethic and crisis mentality stand in the way.
Our everwise Confucius once said 人無遠慮必有近憂. But can't we work hard and be happy at the same time? Not if you work too hard and worry too much, I guess.
Come to think about it, traditional Chinese work ethic and Western Puritan work ethic have a lot in common. In both value systems, not working hard is a sin, a totally silly and absurd idea.