Blessed are the poor in spirit, for theirs is the kingdom of heaven.
Blessed are those who mourn, for they shall be comforted.
You may alread know all of the Beatitudes. (I get them all jumbled together. That's why I've decided I need to purposefully memorize them correctly.) If you don't know them you probably know which verses are refered to when you hear the word "Beatitude." But do you know why they are called that?
If we are humble and appreciate that all of our gifts and blessings come from God, we grow in love and gratitude for Jesus Christ our Savior. But this can only produce mourning and regret over our own sins and the sins of this world, for we have hurt the one who has been so good to us. One also mourns for the suffering of others.
St. Gregory describes another reason to mourn: the more one ascends in meditation of Divine Truth, Beauty, and Goodness, and then realize the poverty of human nature, man can only be left in sorrow. When one contemplates that we were made in the image and likeness of God and lived in Paradise, the Garden of Eden, and compare that to our present state after the Fall, one can only mourn our present condition. But the sentence continues that they shall be comforted, by the Comforter, the Holy Spirit, and hopefully one day in the Kingdom of Heaven.
Mourning in this context is called a blessing, because mourning our fallen nature creates in us a desire to improve ourselves and to do what is right!