I think smart people are told much more often as kids how bright of a future they have. So they build up expectations of "succeeding" in some sense (becoming a doctor, getting rich, etc.). These are the sort of expectations you mention in your quote. Not only is there often pressure put on you if you're smart, you adopt those expectations yourself. Or at least hold yourself to that standard. Of course, being smart doesn't automatically equal success, there are so many other factors. So people often fall short of expectations and feel shitty about themselves and are unhappy. Then there's also the fact that high achievers often hold themselves to unrealistic standards even if they "succeed", so they also struggle to be happy.