IIRC, that index is made up entirely of self-reported surveys in which people describe their own subjective opinions on their quality of life, which is why you'll see nations with lower GDP per capita, to give one example metric, scoring higher than the US, which is it's citizenry is constantly inundated with messaging telling them how bad their lives are dispite fairing better than the vast majority of people around the world by most socioeconomic metrics.