So (married) Republicans are happier with their marriages than (married) Democrats. People with stronger political affiliation are happier than those with weaker ones, with pure independents the least happy. I wouldn't call the differences large, but they are real (the F-statistic with 6df in the numerator is about 27).
Does this pattern change over time? Probably a little, but the general shape remains the same. I broke the data into three periods of roughly equal length:
The most noticeable difference is that average reported happiness with marriage was higher in the first period. The party differences on Democrat to Independent side may have changed a little, but nothing much is visible on the Independent to Republican side.
Why do these differences exist? My idea is that some people are more inclined to be satisfied with things, and they'll be attracted to the party of the status quo (see my previous post). As for why independents are the least happy, I think it's because people who are fatalistic or cynical are less likely to be happy with things, and many independents or weak partisans are fatalistic or cynical--that is, they're basically not interested.
Note: I just got a call from the Ben Carson presidential campaign. Maybe they figure that since I'm happy with my marriage, I'm a good prospect.