The human animal is amazing at making plans and modifying its brain if it wants to. But the human brain and the human animal are also dreadfully bad at doing what's best for us ... Our reward systems are not designed for things that are just good for us; they're designed for things that optimize the progression of our species. They'll grab onto and ratchet into any behavior that makes us feel good, so the human brain is really not optimized for making the best choices.