The World Database of Happiness (in Rotterdam, of all places: logo right) compiles results drawn from happiness surveys taken around the world. Their current summary puts Denmark, Switzerland, Austria, Iceland, and Finland at the top ranks (the Netherlands is 15th and stable; Belgium 18th and dropping). The Economist credits this to being "married, extroverted, optimistic, Republican, religious, sexually active, and a college graduate with a short commute to work."
Happily, I qualify for less than half of these.
There may be another factor to consider. Leopold Kohr, in his 1957 book The Breakdown of Nations, argued that popular misery was strongly correlated with the physical size of the nation. He believed that as countries became larger, governments were less able to connect with constituent members, leading to policies that marginalize individuals. Powerful politicians become arrogant and aggressive, expanding borders and claiming resources from smaller neighbors. Larger nations enclose culturally diverse, sometimes antagonistic groups leading to social stresses.
Kohr's answer was to limit nations to the size of "tribal" units, able to develop local, socially cohesive ways of defining and maintaining culture, law, and economies. As an example, he redrew the map of Europe to show how this might look in practice (50 years later, Eastern Europe has actually evolved this way).
Interestingly, the Netherlands was already optimally sized in this regard: I think this might help to explain it's overall happiness?
Kohr's ideas were later echoed by Schumacher's Small is Beautiful and Callenbach's Ecotopia, and I admit to having a romantic affinity for the overall notion of limited national size, human-scale development, and sustainable economics. I've observed that when political, corporate, or religious organizations become large, their scale enables leaders and their subordinates to pursue their ambitions without challenge. Bad results follow: Simple human concerns are reduced to statistics; social progress falls prey to ever more political and economic growth.
But I've seen it go the other way, too. In Seattle in the 80's, downtown redevelopment interests and civic boosters pressed for Seattle to grow and take it's rightful place as a world stage. Emmett Watson, a columnist for the Seattle Times newspaper, appealingly advocated for Lesser Seattle, a place of neighborhoods and mountain views and Northwest heritage. He prevailed, saving the Pike Street Farmer's Market and the Pioneer Square Historic District. But his battle continues (SeattleWeekly credit, left).
The Dutch are fond of reminding me that the Netherlands is a small country, and also an influential one. I agree. But therein lies the tension: can it be both? If not, then what choice will the country make, why, and how? It's been a great table-topic, with well-advocated opinions on both sides. I think that the Dutch happiness derives from the human scale of the country and the social cohesiveness of the people. So long as they are content to wield influence by example, rather than through growth, I believe that they can continue to have individual happiness as an overall result.