Interesting. All the other evidence I've seen suggest that committed relationships do make people happier, so I'd be interested to see how these apparently conflicting findings can be resolved.
Part of the difference could just be the focus on marriage vs. stable relationships more generally (whether married or not): I'm not sure there's much reason to think that a marriage certificate is going to make a big difference in and of itself (or that anyone's really claiming that it would). In fact, there's some, albeit limited, evidence that unmarried couples are happier on average than married ones.
I'll try to dig up references when I have a bit more time. Don't suppose you happen to have one for the actual research behind your first link?
"All other evidence" being? I a priori doubt all the happiness research as based on silly questionnaires and naive statistics (and most other psychological research). Is there any good metaanalysis showing anything like that?
Whenever the topic of happiness is mentioned, it's always discussed like it's the most important thing in the world. People talk about it like they would a hidden treasure or a rare beast - you have to seek it, hunt it, ensnare it and hold it tight, or it'll slip through your fingers. Perhaps it's just the contrarian in me, but this seems misguided - happiness shouldn't be searched for like the holy grail. Not that I don't want to be happy, but is that really the purpose of my life - to have my neurons stimulated in a way that feels good, and try to keep that up until I die? Why don't I just slip myself into a Soma-coma then? Of course, anything I do boils down to a particular stimulation of neurons, but that doesn't mean there's not something better to aspire to. To pursue happiness as an end itself I think, is backwards. It wasn't built into our brains because evolution was being nice - it's there because it increases our fitness. Happiness is designed to get us somewhere, not to be a destination in itself.