Thus, according to Nachmanides, God insists that we display kindness toward animals, not so much because of His concern for these creatures, but because of His concern for us. If we act unkindly toward a feeling, living creature, we implant a streak of cruelty within ourselves. The flesh of a wild beast, a predator, can have a powerful and detrimental affect upon the character of the one who eats it. In a similar vein, Nachmanides writes that the mitzvot which command us to remember God's miracles are not designed for God's sake, but for our own. By acknowledging God's wondrous kindness and salvation, our hearts are imbued with the positive traits of gratitude and humility and our souls are suffused with love of God.
Every mitzvah improves man.
The Vilna Gaon comments that man was created so that he may eliminate his negative character traits. The person who fails to accomplish this has lived to no avail. One must be more heedful of evil emotions than of observing the positive and negative commands. Upon completion of the building, the builder was paid the stipulated sum, yet the builder wanted more. The landlord, however, adamantly refused to pay.