Treating a being as an end in itself means respecting the value of what makes it such an end. After we see that this value resides in rational nature… - Allen W. Wood

Here is a conceptual truth about reasons: If it is impossible for us to do otherwise, that can never be because there is a reason to act as we do.

Kant’s theory of freedom, and especially the idea that we are free only in the intelligible world beyond nature, has also been the chief stumbling block to the acceptance of his moral philosophy. The scandal has only increased with the passage of time, as fewer and fewer moral philosophers find it tolerable to burden morality with an extravagant supernaturalist metaphysics.

There are two main reasons that Kant refuses to allow that sympathy or any other empirical sentiment or desire could constitute the foundation of morality. One is that no sentiment of this kind can yield the kinds of objective and universal principles that morality requires. They can approximate to this only by claiming a greater empirical uniformity in human nature than experience shows to be there. […] Kant’s other main reason for rejecting sympathy or love as the basis of morality involves his view of the empirical psychology of these feelings as they arise in us in our social condition, and especially in the “civilized” condition of modern European society.