As a Kantian thinker Korsgaard is committed to the idea that our 'true self' is rational throughout. This leads her to the argument that since human beings are rational, they cannot but choose what to do. They must act. They cannot not act. And since they must act - because they are rational - it is only good action if they act rationally. From this, Korsgaard infers that the categorical imperative is the principle of human action and practical reason and autonomous agents.
Even though she is rightly accused of bootstrapping - like the Baron of Münchhausen pulling himself up on his own bootstraps - it is very interesting to see how she interpretes Kant and Aristotle as having in essence the same theory and what it means to act rationally.