when we tell the truth, we have to do so not “because we care about our friend” or “we’re afraid we’ll be caught in a lie” or something—we tell the truth only out of a duty to follow the universal maxim we have reasoned out.
For Kant, an action gets no moral credit unless it's motivated in the right way. And the "right way" is FOR DUTY. If it happens to be the exact thing a dutiful person would do, it still gets zero moral credit if it's not done BECAUSE it's one's duty. (That motivation is necessary, but -- as I read Kant -- it needn't be that one wouldn't also prefer to act that way. It just has to be the case that one would so act even if only for duty.)
