The Light fills the vessels or desires in reverse order (according to the law of reverse correspondence). The Light needs to correspond to the desire, meaning the deeper the desire and the stronger the screen or intention on it, the more powerful the Light that it attracts.
However, this Light does not necessarily clothe into the same desire that attracted it. First, the Light enters the thinnest vessels (desires), which is exactly the opposite of the desire for which it was meant. This causes all the confusion in this world and the mysteries of the Upper Governance.
At the expense of great effort and suffering, meaning by working with the strongest desires, a person acquires a sharper mind and a more refined perception or sensitivity. We see how after going through many blows on our crudest desires, right “on the flesh,” we suddenly begin to feel some new way of thinking within us, on higher degrees.
This is how the law of reverse correspondence of Light and desire works (Erech Afuch Orot Ve Kelim). I receive a blow on my egoism, realization of evil, and from the suffering of this crude and primitive ego, I suddenly begin to better understand spirituality, the Creator. It would seem, what does one thing have to do with the other?
From the 3rd part of the Daily Kabbalah Lesson 11/30/10, Beit Shaar HaKavanot