I think the links between taxation, spending, and inequality are the most plausible explanation of the fact that the highest-taxed countries are the happiest. It can’t be that paying taxes makes Danes happy. But plausibly, living in a relatively egalitarian society makes people happy.
I wrote a paper about this! At the time, the studies showed no notable systematic relationship between income inequality and happiness. (I know Betsey Stevenson and Justin Wolfers are looking at the question again with the bigger, better Gallup survey, so maybe they'll offer a somewhat differnet, more accurate picture.) Danes say they're really happy and have the lowest inequality. But Americans are nearly as happy and have high inequality for an OECD country. Mexicans are a quite upbeat lot, but have really, really high income inequality. So there's not much of a clear pattern in the data. The effect of inequality on happiness appears to be pretty strongly ideologically mediated. Unsurprisingly, high inequality tends to be disquieting to egalitarians. But it doesn't so much bother meritocrats. Additionally, the causes of high inequality are various. Economic predation by political elites (lots of Latin America and Africa) is pretty likely to create a sense of victimization and injustice. But high levels of wealth creation in more or less fair institutions but with relatively little fiscal redistribution (the U.S.) doesn't bother people as long as they think the system is more or less fair. So the national income inequality variable itself tends to have little or no independent effect. The effect it does have depends on other things people believe and care about and the specific causes of inequality in different places. Anyway, why would you expect nation-level income inequality to figure much into an individual's assessment of life satisfaction? People don't experience national Gini coefficients. They worry about their neighbor's car.