Personally I think happiness is something very individual and it is up to everyone individually to decide what will make them happy. This means that there are as many version of happiness out there as there are people and one should be cautious in making assumptions about what makes others happy.
That being said, it is very clear that humans are very social in nature and walking the path of life with someone special can certainly contribute a great deal to someone's happiness. I would venture to say that it doesn't have to be marriage, it can just as well be a 'life partner'. I think that for a majority of the population, a close, intimate and stable relationship can indeed increase their quality of life.
This is not to say that single people are unhappy, but a lot of singles might be missing something - either consciously or unconsciously - that is inherently human: sharing your life with someone very close.
Because of that 'humans are a social animal' factor, I think that indeed the majority of married persons (or people with a stable long term partner) are happier than their single counterparts, with the caveat that, as stated above, happiness is very individual and generally defies generalization.