You don't need to make it that complicated. Happiness is just difference between what you expected and what you got. So a shepherd in Bhutan might expect to his goats to deliver 10 baby goats, but got 15. Then he is happy.
Also, calling USA as most successful country is baseless. Nigeria might have more younger population and better demographic pyramid. Other countries might have better climate and may consume less processed food. People may be leaner in Asia and healthier. Someone might call Americans as obese, unhealthy, gender-confused, less competitive in STEM, politically polarized, sales-oriented conmen. What do you mean by success?
Also, calling USA as most successful country is baseless. Nigeria might have more younger population and better demographic pyramid. Other countries might have better climate and may consume less processed food. People may be leaner in Asia and healthier. Someone might call Americans as obese, unhealthy, gender-confused, less competitive in STEM, politically polarized, sales-oriented conmen. What do you mean by success?