The Creator works only with His Light. The Light affects desires (Kelim, vessels) which, depending on their condition, feel better or worse according to their equivalence to the Light. This is what constitutes the law of equivalence of form.
The more I match the Light, the better I feel. The less I am similar to it, the worse I feel up to the point that I feel a blow. All of these are effects of the Upper Light. Even the darkness I experience is also the Light working on me, and it is only my opposite qualities that reveal Him to me as darkness. Instead of the Light (Ohr), I feel darkness (Orta).
However, the Creator possesses no darkness. Simply, when He adds a little bit of Light, we immediately feel bad. This is because in this Light, we uncover a greater desire, and that’s when darkness falls.
The Light stands against desire. In other words, with the help of the Light, enormous desires unfold in me. Somewhere across from them, in a place concealed from me, there lies enormous pleasures, and, yet, I suffer.
It may also manifest in health problems, lack of sustenance, bad mood, inner restlessness, and even in wanting to die, but it is all the effect of the Light. If the Light fills prepared desires (Kelim, vessels), it is experienced as pleasure since it fills them. If the Kelim are not ready, it is a blow.
From the Daily Kabbalah Lesson 11/26/10, Baal HaSulam, Letter 52, 1928