"If you're so clever, why aren't you happy?" a friend's mother would say to him.
I'd turn that around. I think that for some, being unhappy is what stimulates the mind. That doesn't mean that I believe it's a good thing; it's just that your brain accelerates, looking for a way to survive. Having taught Looked After Children for a couple of years, I'd say that typically, although they were academic under-achievers, they were unusually sharp for their age in other ways. Maybe that's also why girls from broken homes become sexualised earlier.
But the one thing that all this cleverness doesn't address is the root of the cleverness itself. It may seem an impertinence to judge people one hasn't met - though that is the bread and butter of the modern media - but would you regard, say, Stephen Fry or Dawn French as happy, well-balanced people?
A clever person is more likely to out-argue you than to put right anything about themselves. I've read that psychoanalysts find highly intelligent patients the hardest to cure; and that successful entertainers avoid seeking a solution for their neuroses, because it might turn off the tap of their talent.
I hope for a world that is fit, not for heroes nor for geniuses, but for the dully contented.