Tanya Chapter 35 part 3

A mitzvah is different from everything else in our experience. In a world of concealment, the mitzvah represents the total manifestation of the Divine will. In contrast to rapturous feelings of love, or deep feelings of awe for Hashem, a mitzvah is not an outgrowth of the human experience. It is rather pure revelation of the Divine will. The only thing that can manifest the Divine presence is something that is completely dissolved in the Divine light. And that means anything with an identity and consciousness of its own, even if it be a holy soul burning with passion for Hashem, cannot manifest the Shechina –– except through the act of the mitzvah which has no identity other than being the Divine will.