1) Smart children are raised with expectations of doing great things because they are smart. If you have two children and one is obviously smarter than the other, the burden of your hopes and dreams will naturally fall on the smarter one, and equal accomplishments between the two will not have equal praise. Later in life this turns into being unhappy because you feel like you could do more because you are smart. It's a constant burden.
2) Being smart brings with it a certain level of ability to "peer behind the curtain" so to speak, and see just how dirty the real world is. It can make it seem like sinking time and effort to make meaningful changes is just futile because of how many things are messed up. And that kind of mindset saps happiness in a hurry.