This is an interesting hypothesis, but I don't buy it. I'm pretty confident that "smart" people are also good at solving poorly specified problems. One good example might be, "How do I start a successful company?" There's plenty of evidence that intelligence correlates to ability there.

I think the answer is simpler. The introduction basically asks, if smarter people are better at planning and solving problems, why can't they make the choices that will make them happy? And the answer is that humans have evolved to maintain a relatively stable level of long-term happiness, assuming their basic needs are met, essentially regardless of other factors. Getting what you want can provide a short-term boost, but you quickly adapt. Likewise if you suffer a setback, assuming it doesn't permanently impact your ability to meet basic needs, you adapt. Presumably this is because if people permanently became too blissful, they would lose their drive to strive for more resources and mating opportunities. Likewise if someone is depressed. So evolution has tuned us to a middle ground. Intelligence may allow one to understand this, and maybe even to accept it, but not to somehow think their way out of it.