Answer: Our desire determines everything, but the Light does not change. All the changes take place only within a person: where he aims his desire at, and relative to this, what strength he needs. He receives according to this.
We are always under the influence of some light, but it is very weak. We do not match this light, but feel its lack, and that is why we do not notice it.
Light is a force, but it affects us either to the extent of our similarity to it or the opposite. That is why the light of the Passover holiday is a huge light, but it is not manifested openly.
We think that light is when it is light outside. But there is nothing outside—the entire world is inside us. And that is why light means the light of mind, the light of feeling, opening in our heart and thoughts.
A vast world that seems to surround us is revealed, in fact, in our sensations and mind. And Light is the force of revelation in the feeling or thought.
From the 1st part of the Daily Kabbalah Lesson 4/2/12, Shamati 59