Sri Sri Ravi Shankar: Sukh Mein Seva Aur Dukh Mein Tyaag. When you are happy, do some service activities and when you are miserable, you must have the courage to let go.
Problems give depth to your life, while happiness motivates you to serve others. So happiness and sorrow come and go but when we connect to that Divine power that is within us and when we have this faith that God is my own, he is right here with me, then sorrow does not come at all.