After reading The Groundwork of the Metaphysics of Morals, I'm still unsure why human beings have a duty to obey the Categorical Imperative. I understand Kant's argument why a rational will necessarily obeys it, but I do not see how it carries the force of an "ought", and thus why beings not bound by perfect rationality (e.g. humans) ought follow it. My suspicion is that Kant requires a good will to be rational, and since he holds that only a good will has intrinsic moral value the conclusion that we have a duty to obey the CI follows. This seems to be supported by the Stanford Encyclopedia of Philosophy, which states:
In Kant's terms, a good will is a will whose decisions are wholly determined by moral demands or as he often refers to this, by the Moral Law.
This implies that a good will is rational, as according to Kant any moral demand must be completely rational. However, I haven't been able to find strong textual support for this in Groundwork.
Is my suspicion correct? If not, what makes the CI an "ought" rather than just something a perfectly rational will would do?