Depression is the result of not handling our life God's way. His inward hostility toward the Ninevites turned into anger. We may not be responsible for the problems of life, but we are responsible for the way we respond and handle the problems.
Depression is caused by self-pity.
When Elijah thought he was alone in his battle against Jezebel, he indulged in self-pity which led to depression. Self-pity is dealing with life's problems from a self centered point of view. God reminded him that there were 7,000 men in Israel who had never bowed to Baal. God also urged Elijah to get into action to counteract his depression. And, Elijah did, with God's help.