After about a half-century of observing the species, my wishy-washy conclusion is that some writers mine their own lives to such an extent that you wonder whether they could handle happiness -- their muse would dry up for lack of material. "How happy it is to be miserable," quoting the Limeliters' parody of Russian literature; "How miserable it is to be happy."
A corollary that's usually unspoken is the idea that one must suffer in order to create effective art. Ergo, a happy writer is doomed to mediocrity.
I don't think it's universally true, of course, and a large percentage of people who make a good living from writing lead happy and sane lives. This profundity may conceal the subtle truth that people who don't worry a lot about money are happier than people who do.
My own experience is that the two variables, success as a creator versus happiness as a person, correlate only weakly. Too many exceptions on both sides.