What a human being can bear
On the axis: Moral demandingness · human nature
Does a demand beyond our strength breed discouragement and hypocrisy, or must the ideal exceed what we are precisely in order to pull us upward? What morality may ask depends on what human beings can give.
Justification
The moral demand is high: always act from duty and not from mere conformity, subjecting all one's maxims to the universal law. Kant tempers this by distinguishing perfect duties, strict and without exception, from imperfect duties such as beneficence or the cultivation of one's talents, which leave latitude to judgment, holding back from an extreme maximalism.
Groundwork of the Metaphysics of Morals, section II · Metaphysics of Morals, Doctrine of Virtue
Justification
Kant maintains a : there is in human nature a propensity to subordinate the moral law to self-love, and "unsocial sociability" mixes in each a tendency to oppose others. This diagnosis, tempered by the predisposition to good and by perfectibility, leans toward a moderate anthropological pessimism.
Religion within the Bounds of Bare Reason, Part One · Idea for a Universal History with a Cosmopolitan Purpose, Fourth Proposition
leads to: Vision of humanity (How far can human beings be changed?)