The maxim of self-love simply advises; the law of morality commands.
There is a vast difference between what we are advised and what we are
obliged to do. No practical laws can be based on the principle of
happiness, even on that of universal happiness, for the knowledge of
this happiness rests on merely empirical or experimental data, every
man's ideas of it being conditioned only on his individual opinion.
Therefore, this principle of happiness cannot prescribe rules for all
rational beings.
But the moral law demands prompt obedience from everyone, and thus even
the most ordinary intelligence can discern what should be done. Everyone
has power to comply with the dictates of morality, but even with regard
to any single aim it is not easy to satisfy the vague precept of
happiness. Nothing could be more absurd than a command that everyone
should make himself happy, for one never commands anyone to do what he
inevitably wishes to do. Finally, in the idea of our practical reason,
there is something which accompanies the violation of a moral
law--namely, its demerit, with the consciousness that punishment is a
natural consequence. Therefore, punishment should be connected in the
idea of practical reason with crime, as a consequence of the crime, by
the principles of moral legislation.