The dignity of human beings is their potential for actual humanity, their ability to become good persons. All else in the universe pales by comparison to moral goodness. Thus, human beings can think of themselves in two ways. Depending on how one views himself, whether one believes himself free to do what is right or believes he can't help what he does, one can develop or fail to develop his own humanity. Man's belief in freedom or belief in determinism determines his moral capacity.
Duty presupposes freedom.
Thus, ethics requires detachment from all inclination or interest. One's sole attachment is to moral perfection. But duty ignores feeling attachment to other persons. Morality has nothing to do with feeling. We have in Kant the cool, detached humanitarianism of the Stoic.