There are some very basic reasons why conservatives, in general, are happier than liberals, regardless of individual income level. While liberals look at income inequality and see it as the result of a fundamentally unjust system, conservatives look at the same income inequality and see it as the result of merit, such as some people being more intelligent, working harder, etc. than others.
Liberals look at poverty in the United States as a shameful disgrace that proves the fundamentally unjust nature of our society, while conservatives look at America's poor and see that they are well off compared to a vast majority of the world's people. Even conservative poor people tend to be happier than liberal poor people, and because of their positive outlook, are probably more likely to move up in society as well.
These facts are well documented by organizations like the Pew Research Center:
http://pewresearch.org/assets/social/pdf/AreWeHappyYet.pdf
If anything I am saying is not true, please prove it wrong. Watching this video alone should dispel the negative view that liberals tend to have about America's economic system:
Matt Mitchell Explains World Income Inequality - YouTube