I would add a qualification to one theory mentioned. I'm not sure that smart people are on average less happy. Consider that intelligence and liberalism have been closely associated in thousands of studies on personality, social views, politics, etc; as both seem to be expressions of the common factor of the dual personality trait openness/intellectuality. 

I guess this fits my experience, if biased. I'm above average in intelligence (according to IQ tests since childhood), have diverse intellectual interests, and constantly read difficult material. So, I'm 'learned' in that I know a little bit about a lot of things, though a college drop out; i.e., an autodidact. I was raised by college-educated professionals, both with teaching careers, one a public school teacher and the other a professor. That meant I had instilled in me a love of learning that, obviously, I don't personally identify with and limit to higher education.