Why are rational beings ends in themselves? (Kant)?

I am trying to better understand Kant's argument in the Groundwork of the Metaphysic of Morals and I have a couple of questions. Why are rational beings ends in themselves? Is it because of a capacity for reason, or is it because of the good will, or is it because of something else? If rational beings are... show more I am trying to better understand Kant's argument in the Groundwork of the Metaphysic of Morals and I have a couple of questions.

Why are rational beings ends in themselves? Is it because of a capacity for reason, or is it because of the good will, or is it because of something else?
If rational beings are ends in themselves, does that imply that non-rational beings, like animals, cannot be ends in themselves?

Thanks to everyone for helping!
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