Each year the OEDC publishes a study which examines the average level of happiness amongst citizens of various countries. They measure 11 key data points" "housing, income, jobs, community, education, environment, governance, health, life satisfaction, safety and work-life balance".
The results are surprising and revealing. One might expect the US. to do well yet we don't even place in the top ten. Denmark wins almost every year. Lots of reasons for this are obvious; the state is well-run, equitable, with good work life balance and a high standard of living. But the big secret may lie in how they view the Future -- they have low expectations and therefore are usually pleasantly surprised how well things turn out.
Hans Rosling's astonishing video below tells most of the story as quickly as possible. Broadly, it makes sense that table-stakes for Happiness are health and wealth. But there's more to it than that.