This is a very poorly designed poll, and if it really is a Gallup poll, I'm disappointed in them. That list includes all sorts of things that appear to be proving something, but in fact prove nothing.
A higher proportion of Republicans are satisfied with their lives than Democrats? Well, how about if you ask whether a poorer group of people tend to be less satisfied? (And, big surprise, a larger percentage of that group are Democrats?) What does their political affiliation have to do with whether they are satisfied with their lives?
OK, maybe rich vs. poor doesn't prove it, how about white vs. minority groups? More of the minorities are Democrats, so does that drag party affiliation into the mix? You might as well say, "People with blue eyes are happier." And then, out of that group with blue eyes, see how many of them are Republican, vs. people with brown eyes. Therefore, being Republican makes them happier?
If this really is a Gallup poll, they've come a long way down in my estimation. Maybe it's just an attempt to make a ridiculous political point. Or, if all those factors are just random, it's not really a "poll," it's a mishmash.