Clinical depression is not the Dark Night of the Soul because it is caused by chemical imbalances in the body or external events such as the death of a loved one, that cause us sadness.
Clinical depression is usually characterized not only by sadness but also by anxiety, feelings of helplessness and pessimism, loss of interest in pleasure, decrease in energy, difficulty in concentrating and remembering, loss of sleep and appetite, thoughts of death and suicide.
Dante begins his Divine Comedy with these words:
along the journey of life
I woke to find myself in a dark wood,
for I wondered off the straight path. ...
a bitter place! Death could scarce be betterer."
(Divine Comedy: Inferno, Vs 1,3)
The Dark Night of the Soul is caused by God. Every human being has disordered desires and attachments. We seek our own comforts and pleasures rather then God's will. The vision of God can only be obtained by burning away the imperfections of our soul. Even spiritual consolation may be taken away allowing us to love God only with the naked intent of the will. The Dark Night is purgatory in this life. The soul may feel abandoned by God. The last cry of Christ on the Cross, "My God, My God, why have you abandoned me." (Mt 27:46) now is understood. Of course, God never abandons anyone, but the experience of dying engenders a feeling that we are returning to nothingness. "During the dark night, God roots out our deepest attachments to sin and self, and the desolation that accompanies that rooting out is overwhelming and crushing." (osv.com/ Emily Stimpson, Understanding the 'dark night of the soul')