We desire things for reasons. Often we want something because we judge it to be good. The desire thus stems from an evaluative belief, which in turn is answerable to reason. In other cases, the reason for our desire is a brute taste, e.g. the fact that we find the taste of chocolate to be pleasant. We desire chocolate for the pleasure it brings us, but this taste (unlike our values) is not further answerable to reason. It is simply given.
Is it possible to have a brute desire? A desire for chocolate, say, not for the pleasure it may bring, nor because there's anything about it you judge to be valuable, but simply because you (inexplicably) want it? I submit that this is not possible. You may be able to program a creature to pursue certain ends for no reason, but it is not really an agent with desires in the fullest sense unless it could make sense of them on reflection.
Consider it from the perspective of the agent in question. You find yourself pursuing some object X, even though you see nothing redeeming about it. You have no taste for X (it does not bring you pleasure). Nor do you judge it to be good or valuable for any other reason. You are, in short, completely indifferent to it. And yet there your hand is reaching out for X. Why is your hand doing that? You can't make sense of it. The behaviour does not stem from your attitudes in any way you can make sense of. It would seem more like your body is possessed, moved by some compulsion outside of your self. Whatever your behavioural outputs, so long as you neither value X nor have a taste for it, there's no way your pursuit of it can be properly characterized as reflecting a "desire" on your part.
The situation seems even more odd when we consider that the alleged "desire" is supposed to govern not just bodily movements, but also internal deliberation. That is, you should find yourself engaging in instrumental reasoning, forming elaborate plans about how best to obtain X. But again: why are you reasoning in such a way? It makes no sense, unless you think that X is worth getting for some reason (hedonistic or otherwise).
If you have a genuine desire that P, then (ceteris paribus) you will accept an offer to make it the case that P even on the condition that this fact is wiped from your memory and so you never get any subjective satisfaction out of it. (If you don't accept this offer, that shows that what you really desire is not P per se, but rather the subjective satisfaction of believing that P.) But, in the above case, can you imagine voluntarily accepting such an offer to secretly bring about X? Such a choice seems incomprehensible (unless you actually valued X, contrary to our stipulated set-up). So brute desires are likewise incomprehensible.
Or am I missing something?