The reason it feels like work to study for a test or solve a puzzle is because, after a certain amount of time, the brain is trying to get you to stop. This is not to say that the brain does not want you to focus, however. Studying and the like are long-term investments. But as we can see from Jim’s example, too much of a long-term investment can be a bad thing. We need to balance long-term investments with short-term gains. Thus, once Bob finds some food, he may be willing to climb that tree again.
Read more and the full theory over here.