The spiritual blindness that happens in the night of the spirit happens because the divine light of God is brighter than the eyes of our soul can handle. This is one reason the night of the spirit hurts — because our souls, being human, are much weaker than the brightness of the divine light of God.
John of the Cross says this:
“The light and wisdom of this contemplation are so pure and bright and the soul it invades is so dark and impure that their meeting is going to be painful. When the eyes are bad — impure and sickly — clear light feels like an ambush and it hurts.”
There’s another reason the night of the spirit is so painful, though, and it’s because what the soul is able to see when the divine light shines upon it are all its imperfections.
The saint describes it this way:
“Consider common, natural light: a sunbeam shines through a window. The freer the air is from little specks of dust, the less clearly we see the ray of light. The more motes that are floating in the air, the more clearly the sunbeam appears to our eyes. This is because light itself is invisible. Light is the means by which the things it strikes are perceived.”
The light of God is a sunbeam on the soul, and our native imperfections are dust motes and particles floating through the air, now clearly visible because of that ray of light. The sudden, acute awareness of all these imperfections makes the soul in this place feel quite wretched.
Remember, the soul that has entered the night of the spirit has already endured the night of the senses. Her love for God has been purified a great deal, and she has come to a place of being wildly in love with God.
Seeing her impurities through the searing light of God undoes her.
She feels these impurities will separate her from the lover of her soul, God, forever.