My impression came from a while ago when Denmark came out on top of one of those rankings and some of the other recent winners had been Central American countries with lots of strife. It had people wondering what the hell "happiness" was picking up.

There were some interviews with Danes about it and one of the things that came through is a culture of relatively low expectations and therefor low disappointment.

Can't say I've ever heard Undisc's explanation but perhaps. 

My fav candidate explanations are
- social cohesion/cultural community 
- an economic and social/political system that cuts out the worst outcomes (even at extreme expense to others) which improves the average of these reports 

- liberal distribution and use of antidepressants (and to the extent they work etc), which achieves the same thing: statistically cut out the worst, most miserable peeps

But yeah, it's always fun to present number-1 status to Finns or Danes and see them go, _huh, wth are you talking about??_