This presentation I found over on Asi's always excellent private selections, pulls together some things I've been talking / thinking about for ages, more eloquently and usefully than I ever could. It's great.
Go steal it.
The more I think about this, the more I'm convinced that we are in the happiness business - that it's our job to help people be happy, which seems like a good thing.
I suspect that things that make people happy exist at equilibrium points - more of something [x] makes you happier, up to a certain point [y], after which the same thing will make you less happy.
This has been demonstrated in arousal terms, leading to the development of the Wundt curve, which shows hedonic value garnered from increased arousal drops after a certain point.
Which, I guess, bring us to the the platitude that too much of a good thing, ain't.