What kind of mourning is this that is central to the life and "growth chart" development of a disciple? It's about being brought to a recognition of our deep spiritual poverty by a moving experience of sin's effects (on us, on others, on God). We mourn those effects, and God uses this to keep us from falling back into sin and to form us into disciples who have his heart for sinners, that they, too, would find his great forgiveness.