Mourning is more than sadness. It is the gut-wrenching wail of devastation and loss. It is the same Greek word used is Revelation 18 when Babylon falls and the people realize there is nothing left of the great city. Mourning is the attitude of the prodigal, the repentance of Job, and the cry of David at his lowest moment in life (Luke 15, Job 42, Psalm 51).
Mourning is a present and active response to sin, and not just our individual sins, but over the ramifications of human sin on those who live in this world without hope. Mourning laments over the divisions in the Church, the oppressive treatment by one person or group over another, and the chaos that comes from human nature run amok.
Jesus offers consolation to mourners, telling them that they will be comforted. Mourning is not done in vain when it causes us to turn to the One who is the Comforter.