The greatest reward is the understanding that despite man’s lowliness, God desires his service and derives satisfaction from it, and the actions of man are extremely significant.

This is the ladder firmly planted in the ground to ascend the mountain of God and stand in His holy place. For it is through [first performing mitzvot] not for their own sake that one comes [to perform them] for their own sake, as he will understand in his heart that it is fitting to serve God not in order to receive reward, but rather to bring Him satisfaction. For there is no greater reward in this world than the fact that the great, lofty, elevated, terrifying, and awesome King consents to man, who is like nothingness and vapor, serving Him, and that a man’s actions are considered desirable and satisfying before His throne of honor. What honor and greatness a man attains when a flesh and blood king desires him and lifts him up from the dirt to stand and serve him, finds satisfaction in his service, and enjoys [his company]! How infinitely and immeasurably greater it is when the King of all kings, the Holy One, blessed be He, desires us, derives satisfaction from our actions, and builds entire worlds from our good deeds! If this were our only reward it would be more than enough. (Rabbi Eliezer Papo, Peleh Yo’etz, Sekhar Mitzva)

When Rabbi Yisrael of Rizhin was incarcerated in a Kiev prison, he was greatly aided by two Jewish jailers. They had been drafted into the Russian army in their youth and considered it meritorious to help the tzadik. When he was freed, he wanted to bless them and asked them how they wanted to be blessed. One of them said: “I know that I have not been involved in either Torah or mitzvot, but, Rebbe, please bless me that in merit of what I did I should have a share in the World to Come.”

“The reward of a mitzva is a mitzva” (Avot 4:2). This means that the greatest possible reward for a mitzva is the mitzva itself, the connection and attachment with the Holy One, blessed be He. (The word mitzva is related to the word togetherness [tzavta].) (Rabbi Menaĥem Mendel Schneerson, Torat Menaĥem 2, p. 124)

Further reading: A proper view on human suffering makes it easier to experience joy; see p. 228.