Though everything else may be going great, we suddenly get unhappy with the raise a colleague on the next table may have got if somehow in our mind we believe that he does not deserve it. Likewise, if we believe that the Boss above somehow does not like us, doom and gloom prevails in our world from the very moment of such realisation. The internally self-triggered unhappiness continues within us for a long time.
Nature and the world outside may thereafter change but unless the perspective within and your internal vision of the world is not changed, you will continue to remain unhappy.
Happiness thus they say is embedded within us. Deeply. Only resetting your perspective will change your happiness