A dark night of the soul is a kind of initiation, taking you from one phase of life into another. You may have several dark nights in the course of your life because you are always becoming more of a person and entering life more fully. At least, that is the hope.
One simple rule is that a truly deep dark night requires an extraordinary development in life. One outstanding example is Abraham Lincoln. With his early life surrounded by death and loneliness and his adult life weighed down by a war in which thousands of young men died, he was a seriously melancholic man who, in spite of or through his dark night, became an icon of wisdom and leadership. One theory is that he escaped his melancholy in his efforts for his country, but another possibility is that the very darkness of his life—he once said, “If there’s a worse place than hell, I’m in it.”—was the ground out of which his leadership grew.