Malbim: Not only does Hashem pay a person based on his acts. Further, it is not according to Hashem's way (his Avodah should be proper for Hashem's grandeur, which is without end). Rather, it is based on man's ways - he is physical, weak, and has a Yetzer ha'Ra. If so, one who is pleasing to Hashem, why does he not receive reward? We must conclude that He handed over conduct of individuals to the Ma'arachah.