The Reward for a Mitzvah Is a Mitzvah

The Kesav Sofer explains the Mishnah "the reward for a mitzvah is a mitzvah" using two other statements of Chazal. One is that whoever is greater than his fellow has a greater yetzer hara (Sukkah 52a). The other, in a Mishnah in Pirkei Avos (5:23), is that the reward is according to the pain and effort. A person whose yetzer hara urges him to sin, but who overcomes it, earns great reward, and this makes the mitzvah greater, since he is rewarded in proportion to the strain and difficulty of mastering his desires. That creates a cycle. Each mitzvah makes him more righteous and raises him to a higher level, which makes his yetzer hara stronger and more formidable the next time. So each following mitzvah is a greater accomplishment than the one before. This is how the Kesav Sofer reads the Mishnah: the reward for the mitzvah already done is that the upcoming mitzvah will be even greater.