I believe that technically you are not meant to be happy. At least not for extended periods of time.
Happiness "informs" your psyche that you are surviving well. That is it's primary, if not only, function. The intrinsic meaning of "survival" is to constantly strive to be better than before...ALWAYS without exception.
Thus when happiness is attained then slowly novelty will fade. You may start to take things for granted again or become lazy/comfortable in your new circumstance. You will then desire more once again.
Somewhere in there perhaps is a hidden formula for the right balance. But to expect to be happy all your life is not realistic as you cannot always be in a state of advancement, usually there is a threshold or real life circumstances in the way.
There is an alternative way, and that is to fool your psyche into believing it is happy. i.e. Drugs

But this is also not sustainable and comes at a price.
Disclaimer: This is simply my personal understanding that is an extension of evolution into the way the human psyche works. It could be BS.