A hypothesis: Happiness and success are inversely correlated, because as people become more successful, their expectations of
future success rise faster than the actual reality does. And, as James Hong (HotOrNot.com) famously pointed out, "happiness = reality - expectations". Middle-aged people are the least happy because they are the most outwardly successful, while children and old people are the most happy because (on a recent basis) they are the least outwardly successful.
A question: given all the positives to happiness that the article mentions - better health, higher productivity, etc. - how can we be happier before we get old? It'd suck to have to wait until I'm 70 and my body is falling apart before I'm happy.