A recent report by the OECD shows that from all OECD countries, more people in Iceland, Ireland, Canada and the Netherlands are happy with all four countries with more than 95% its (surveyed) population replying that they are "quite happy" or "very happy".
However it is the Netherlands with more respondants saying they are "very happy". However most of the other northern European countries- i.e. Iceland, Norway, Sweden, Finland and Denmark- are not that far behind with highest percentages of the population saying they are happy.
Why is this?
They do all have the "developed welfare state" although they also have bad/cold weather, and looong winter nights.
However, they also have high suicide rates as well - for example Finland has one of the highest suicide rates and for youth suicides Iceland has the highest of all OECD.