"Unmarried twenty-somethings are more likely to be depressed, drink excessively, and report lower levels of satisfaction than their married counterparts" Let's all say it together: Correlation is not causation.
As I posted elsewhere, you might be interested in some strong evidence for the causal relationship as described by economist Bryan Caplan: http://econlog.econlib.org/archives/2013/03/22_short_argume....
I've seen elsewhere that financial troubles often herald divorce. If the only marriages that can last are ones where someone's making a premium, then of course you'll see one.