||Chumash: Lech L'cha, Shevi'i with Rashi.
|Tanya: R. Isaac Luria (p. 557) ...side of evil." (p. 559).
Mitzva is an idiom of tzavta - "joining," "attachment." Whoever performs a mitzva becomes joined to the Essence of G‑d - may He be blessed - Who issues that particular command. This is the meaning of "The reward of a mitzva is the mitzva (itself)": His becoming attached to the Essence of the En Sof Who ordained the command, is itself his reward.
The above can be understood through an analogy from the physical world: An extremely simple person has an inner sense of bitul (nullity) before the wisdom and greatness of a scholar - a bitul in which he senses himself to be an utter nonentity. The sage in turn, does not sense or perceive the simpleton as belonging at all to the category of human beings! Not that the scholar dismisses or rejects him, G‑d forbid, for that would be an evil trait; he simply sees no connection or relationship with him whatsoever.
Now, when the sage instructs the simple man to do something for him, that command brings the simpleton "into being." In his own self-perception he is no longer a nonentity but a "somebody"; he has assumed a status unto himself in that he is able to carry out an order of the sage, and it is him that the sage addressed and instructed. In the eyes of the sage too, the simpleton now "exists"; he is a "somebody" to whom he (the sage) can speak and instruct. What is more, the command actually unites the lofty, exalted sage with the gross simpleton. The analogue is obvious.
It is understood that in the above analogy there is no difference at all what the command is about, whether a great, lofty matter or a simple trivial one.