Blessed are those who mourn, for they shall be comforted.
Matthew 5:4 TLV

Mourning for one's self, for the oppression of the weak, for the desecration of God's name and temple, for immorality. One might ask why not spare them their mourning in the first place, but the book of Job and the letters to the seven churches in Revelation show that the rewards of the faithful exceed the losses that brought on their mourning.