Sam Peltzman (Chicago-Booth; Google Scholar), The Socio Political Demography of Happiness:

Since 1972 the General Social Survey (GSS) has asked a representative sample of US adults “… [are] you …very happy, pretty happy, or not too happy?” Overall, the population is reasonably happy even after a mild recent decline. I focus on differences along standard socio demographic dimensions: age, race, gender, education, marital status income and geography. I also explore political and social differences. Being married is the most important differentiator with a 30-percentage point happy-unhappy gap over the unmarried.

Income is also important, but Easterlin’s (1974) paradox applies: the rich are much happier than the poor at any moment, but income growth doesn’t matter. Education and racial differences are also consequential, though the black-white gap has narrowed substantially. Geographic, gender and age differences have been relatively unimportant, though old-age unhappiness may be emerging. Conservatives are distinctly happier than liberals as are people who trust others or the Federal government. 

All above differences survive control for other differences.

Um, The Best Predictor of Happiness in America? Marriage:

Americans who are married with children are now leading happier and more prosperous lives, on average, than men and women who are single and childless. …

This truth is borne out yet again in new research from the University of Chicago, which found that marriage is the “the most important differentiator” of who is happy in America, and that falling marriage rates are a chief reason why happiness has declined nationally. The research, surveying thousands of respondents, revealed a startling 30-percentage-point happiness divide between married and unmarried Americans. This happiness boost held true for both men and women. …

The bottom line is that the United States is increasingly riven when it comes to happiness between the haves and have-nots, in large part because record numbers of less privileged Americans are not succeeding at getting, not to mention remaining, married.