It seems that the mental challenge level of a task doesn't just affect my brain's regular reward system, because if that was a case, the stimulants I take for ADHD would be be sufficient, the dopamine would make even the most routine task satisfying.
There is another system, engaged in problem solving, learning and curiosity, involved.
One analogy would be to use a McLaren as a family car. Picture gunk collecting in the valves because they're never blown out.