This speaks to a common fallacy: the notion that happiness is a permanent achievement. Many people believe that you work hard so eventually you can achieve a position where you live "happily ever after". That's however not how the brain works. Happiness is a reward mechanism for effort bearing fruit. Once you stop doing things to become happy, eventually the happiness goes away because the brain adapts to its situation (this also works in reverse, you can get used to surprisingly horrible situations). The trick to being happy all the time is doing new things that make you happy all the time.