IV. The blessedness of mourning discussed
After the assertion of this most happy humility, the Lord has added, saying, “Blessed are they which mourn, for they shall be comforted.” This mourning, beloved, to which eternal comforting is promised, is not the same as the affliction of this world: nor do those laments which are poured out in the sorrowings of the whole human race make any one blessed. The reason for holy groanings, the cause of blessed tears, is very different. Religious grief mourns sin either that of others' or one's own: nor does it mourn for that which is wrought by God's justice, but it laments over that which is committed by man's iniquity, where he that does wrong is more to be deplored than he who suffers it, because the unjust man's wrongdoing plunges him into punishment, but the just man's endurance leads him on to glory.
Source: Sermons (New Advent)