A plausible theory of desire is that x desires A if and only if x is disposed to pursue A (perhaps we should add "as such", to get around Daniel Stampe's worries, or maybe do a functionalist tweak on it and add some "typically" qualifiers). Now it seems that I am disposed to pursue A explains why I pursue A but does not directly justify or give reason for pursuing A. (It could indirectly do so if, say, I promised you to act on my dispositions in some case, or if my therapist told me that it would be good for me to act on more of my dispositions.) Moreover, dispositions to pursue are precisely the sort of thing that itself calls out for reasons. So even if desires, on this view, were reason-giving, that would only be shifting the bump under the rug in an unhelpful way.
That said, there is a view on which one could hold fulfilling because it is good for an entity to be active in accordance with its nature, and it is in the nature of desiring beings to act on their desires. On this Natural Law view, one could hold to something like a dispositional theory of desire (with teleological tweaks) and still think that desires are reason-giving. But it would be very odd to think in a case like this that desires are the only reason-givers. After all, there are other ways of being active in accordance with our nature.
The main alternative to dispositional theories of desire is to see desires as an awareness of, belief in or attention to normative (putative) states of affairs, such as there being a reason to do something or something's being good. On such a view, the reason-giving force of desires is parasitic on the reason-giving force of something else. In fact, this is true in the Natural Law view I offered above, too.
So it really does seem very plausible that if desires are reason-giving, their reason-giving power is parasitic on the reason-giving force of something other than desires.