Where's the Fire?
A rooster has a crest of head feathers. They flatten when he loses a cockfight. That's the origin of the word, "crestfallen."
To be "crestfallen" is to be disconsolate or discouraged; and this state of mind can even devolve into a dark form of despair, while the depressed person is not even aware of the reason for it. The psalmist in a triple refrain asked the question: "Why are you downcast, O my soul" Why so disturbed within me?" He then proposes to himself the antidote: "Put your hope in God, for I will yet praise him, my Savior and my God" (Ps 42:5,11: see Psalms 43:5).
By putting his hope in his "savior and God," David implicity referred to the role of the expected Messiah as a depression healer, for a major part of the messianic mission of Jesus was "to bind up the brokenhearted" (Is 61:1) and "to release the oppressed" (lk 4:18).
From a mild case of the "Monday morning blues" to the serious state of suicidal depression requiring psychotherapy, there is always available for the "crestfallen" the loving support of the depressed Man of Sorrows, who in the Garden of Gethsemane was "overwhelmed with sorrow to the point to death" (Mt 26:38). Yet, paradoxically, it is the same "Man of Sorrows, familiar with suffering" (Is 53:3), who lovingly prescribes an antidepressant: "my joy...in you, that your joy may be complete" (Jn 15:11).