"In the absence of measurement and contraction, it is impossible to receive the vitality from G-d." (Rabbi Nachman of Breslov, Likutey Moharan I, 56)
Comment: The source of vitality of our soul is in G-d. However, the Creator is a source of spiritual light too intense to be received by humans. That is why the Divine Presence contracts Herself to allow us to receive It.
Too much light dazzles. Hashem's greatness is of a nature which is beyond our ability to contain it. Thus, G-d "contracts" Himself in the mitswoth He gave us and which allow us to receive His light safely.
Beyond the mitzvah itself (tefillin, prayer, tzedakah ...), the most important aspect is that of spiritual light that it allows us to receive. It is this light that is our life force and that we must try to increase daily.