Sadness, seriousness are parts of a psychologically sick man - they need causes. So when you are feeling happy, don't start asking, "Why am I happy?" When you are feeling sad ask why you are sad. But strangely, it has become conventional to our minds that when we are sad we accept it as if it is our nature. And when we are joyous even we are surprised; deep inside we even start worrying: "What is happening to me?"