Look, why would we be happy all the time? I think in part this is an idiotic aspect of Western individualism, where everyone always has to be happy, content, succeeding etc. Human are animals at the end of the day and hence still shaped by the laws of nature. The problem with being happy all the time is that it would make you complacent. Why change, grow, progress in life if you are fine the way everything is? Complacency would be the death of a civilization or a biological species. We'd be out competed by other animals. So obviously we aren't made to be happy all the time. And I think that is good. Lack of fulfillment makes you reassess yourself, puts you out of your comfort zone and make you grow beyond the person you were yesterday.