What is your passion in life?
By passion, I mean a purpose that you actively contribute towards (or believe you are doing so). Something that makes you angry when it seems like your cause is losing.
I guess I'm being very abstract here. But my main point is that life feels somewhat empty for a lot of people when the high end purpose is to merely "satisfy/fulfill themselves". I believe that truly happy people need to have a sense of duty towards something beyond just themselves.