These [normative] skeptics usually presuppose a kind of naive Humeanism, according to which preferences are 'given' and automatically combine with beliefs to yield action. But that can't possibly be right, because it leaves no room for the familiar phenomenon of deliberation. We are agents with the capacity for practical reasoning, i.e. the assessment of reasons that count for or against various courses of action. This is a self-consciously normative process of decision: just as theoretical reasoning addresses the question what should I believe?, so practical reasoning addresses the question what should I do? Insofar as you think of yourself as a rational agent at all, you must be engaging with these normative questions; the alternative is to be a mere automaton, a reflexive stimulus-response machine. Most of us are more deliberative; but deliberation is inherently normative: it addresses a question for which there may be better or worse answers.
Robin Hanson responded that he has no problem reflecting on what he wants. But I think that is insufficient to capture all practical deliberation.
One way to bring this out is to note how constrained deliberation on Robin's question must be. If our 'wants' or desires are given prior to deliberation, then an answer to the question 'what do I most want?' is limited to the contents of your pre-existing desires. Your ends are already fixed; all that remains to be determined is which desires are strongest (a mechanistic process which does not seem to call for any kind of choice or decision in any case), and then the instrumental question how to fulfill them.
But as agents with fully general rational capacities, it is possible for us to engage in open-ended deliberation. Our practical conclusions are no more determined by our prior states of desire than our theoretical conclusions are determined by our prior states of belief. (Though both will be of significant influence, for sure.) Regardless of my prior ends, if you can convince me that some new end is more worth pursuing, I must -- on pain of irrationality -- come to adopt that end myself.
The desire theorist may insist that this just shows that you have come to have a new desire, and it's still this desire which is responsible for motivating your eventual action. But this is an empty claim: the desire was a product of your rational deliberation, rather than an input to it. So it is your reason which is the ultimate source of motivation in this case.