Richard (Lord) Layard, a Prof at the London School of Economics has a new book out on the subject of happiness. Specifically answering that question of why, as we all get materially richer, we’re not all getting happier. As Andrew Marr says in today’s Telegraph:

But the main point of the book is Layard’s attempt to answer the question why, as Western societies have become so much richer, we haven’t become happier.
It seems that, once we have enough to live a decent material life, we adjust to think we “need” almost everything we now have. Only relative success brings happiness. And the benefits are real. Oscar winners live, on average, longer than runners-up. More striking still, winners of Olympic silver medals come home less happy than winners of bronze medals. Why? Because silver medallists compare themselves with the golds, which they didn’t quite get, while bronzes compare themselves with the majority who won no medals at all.

I’m sure that the full book delves rather deeper into this but even just the above simple explanation can be used politically by either side.
Egalitarians will emphasise that after a certain level, material goods don’t provide happiness, so greater income re-distribution should increase the sum of human happiness by bringing more or all to that basic material level. Others will point to the importance of “relative success” and argue that as happiness is based on doing better than others, an egalitarian society is apparently against the very basic human drives.
Something for everyone then.