Last month The Atlantic looked at happiness, following the longitudinal study of 268 men who entered Harvard in the late '30s. The author was quite careful not to confuse correlation with causation, but a running theme was that ones happiness is related to the richness of his or her network of friends.
Here's another take on the subject. This study compared nations and relative happiness, and found that Denmark is the happiest country in the world.
Why Danes are Smug: comparative study of life satisfaction in the European Union.
The researchers looked at several factors. The considered the "blonds have more fun" theory (giving credit to Rod Stewart for the discovery?), genetic differences, and cultural aspects. (Money quote: "Meals in Denmark can be politely described as unmemorable. “Danish cuisine” is an oxymoron, except perhaps the open faced “butter breads” that accompany the beer and aquavit Danes consume for lunch.")
Their conclusion is that Danes are happier because they don't expect much.
“Happiness equals reality minus expectations.”
Not super-inspiring, and certainly not going to show up on one of the motivational posters so popular in the cubicle farms and in corporate management circles. But hard to argue with.