Kingdom of ends

I'm reading through Groundwork of the Metaphysics of Morals and not quite grasping what Kant is saying in that every rational being is an end in themselves.

Was his support for this claim in his discussion of objective ends or are we supposed to take "rational being" as an end in itself as true so far as we're being charitable in following along with his framework for morality? Or, so as to avoid a false dichotomy, was there something else that I'm simply missing?