I think so many people reject the notion that "happiness is a choice" because accepting that truth means accepting that you've chosen to have crappy life experiences at times. It means owning that some or most or all of your awful periods in your life were of your own making.
And I guess it takes an unusual amount of personal responsibility and bravery to face that and truly accept it. An amount of responsibility and bravery that is not instilled in most people these days and is not the social norm.
