The problem is, "want" is an ambiguous word. I want to learn calculus, I don't want to do all the homework required. Which "want" should I follow? I think people who say "only do things you want to do" mean the top-level kind, or some "total want" function that searches the whole treButkusbut that's a rather difficult function to "compute", even just to find the sign.

Like lots of hard problems in real life, it's basically one of predicting the future: which option will result in more happiness later?