"Happiness in intelligent people is the rarest thing I know."
ā Ernest Hemingway, The Garden of Eden
In our case, we can see the things that are so deeply wrong with the world (beyond greed and a lack of empathy) while being powerless to change them. And we can't just ignore those things either. It's pure misery. The most intelligent people we have ever known were all at the least really depressed even if they hid it well. Most were suicidal to some extent. Meanwhile those barely on the other side of the curve were so much happier and able to live what seem to be fulfilling lives in ignorant bliss.
It isn't exactly causation, but there certainly appears to be a trend.