"Blessed are those who mourn, for they shall be comforted." (Matthew 5:4)
Mourning is foundational to a Christian life. When Christ says, "Blessed are those that mourn", He isn't referring to "boo-hoo we are all sinners" or "poor me I'm suffering". Christ is referring to the sorrow that accompanies a realization of lost fellowship with God through sin. Something like this: “Woe is me, for I am undone! Because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips; For my eyes have seen the King, the Lord of hosts." (Isaiah 6:5)
Even after we have been comforted by a restored relationship in Christ, the sorrow of every earthly loss is understood to be a derivative of lost fellowship with God. So also, every new loss and sorrow reminds us final comfort and restoration will not be complete until with the Lord. Mourning, by grace, leads again and again to true comfort in Christ's completed work.
If mourning is only of the "boo-hoo" or "poor me" variety, the resulting comforts are temporary counterfeits. And if mourning leads to vengeance, it is only a lazy, cowardly and ineffective way to grieve that guarantees more sorrow with no comfort at all.
