Critics of Kantian ethics sometimes complain that the concept of a categorical imperative makes no sense because there could be no reason for obeying such an imperative. This is usually because they think that the only reason for obeying an imperative must be an end in the sense of an end to be produced. They do not notice that Kant's concept of an objective end in itself is precisely his answer to their question.
Kantian Ethics (2008) - Ch. 5. Humanity