Summary: Those who are blessed are those who mourn. Mourning is hateful and irksome to the human nature.
The preaching of the King – Part 2
Those who are blessed are those who mourn. Mourning is hateful and irksome to the human nature. Jesus said the happy people in this world are those who mourn. If they mourn, how can they be blessed? Only the child of God has the key to this paradox, for "happy are they who sorrow" is at complete variance with the world’s logic. Men have, in all places and in all ages, deemed the prosperous to be the happy ones, but Christ pronounces blessed those who are poor in spirit and who mourn.
It is obvious that it is not every form of mourning Jesus is referring to. There are thousands of mourners in the world who do not come within the scope of this verse. There is a natural, a sinful mourning which is a disconsolate and inordinate grief, refusing to be comforted, or a hopeless remorse like that of Judas when he betrayed Jesus. There is a "godly sorrow," of which the Holy Spirit is the Author.
The "mourning" Jesus is referring to is a spiritual one. The previous verse indicates clearly the line of thought here: "Blessed are the poor in spirit, for theirs is the kingdom of heaven." "Blessed are the poor," not the financially poor, but the poor in heart: those who realize they are spiritual bankrupt the opposite of the Laodicean which says, "I am rich, and increased with goods, and have need of nothing." In like manner it is spiritual mourning Jesus is referring to. Further proof of this is found in the fact that Jesus pronounces these mourners "blessed." They are blessed because the Spirit of God has wrought a work of grace within them, and they have been awakened to see and feel their lost condition. They are "blessed" because God does not leave them at that point, "they shall be comforted."
Mourning Jesus refers to is the initial mourning which precedes a genuine conversion. There must be a real sense of sin and a godly sorrow before the remedy for it will even be desired. Thousands acknowledge that they are sinners, who have never mourned over the fact. The prodigal in Luke 15 (quickview)  before he left the far country said, "I will arise and go unto my Father and say unto Him, Father, I have sinned against heaven, and before you, and am no more worthy to be called your son." The publican of Luke 18 (quickview)  "smite upon his breast" and said "God be merciful to me a sinner"? The prodigal and publican felt a sense of sin in their heart.
The mourning Jesus is referring to springs from a sense of sin, from a tender conscience, from a broken heart. It is a godly sorrow over rebellion against God and hostility to His will. In some cases it is grief over the worldly things the heart has trusted, over the self-righteousness which has caused complacency. It comes from an agonizing realization that it was our sins that nailed Jesus to the cross. It is these tears and groans which prepare the heart to truly welcome and receive the Savior. It is mourning over the felt destitution of our spiritual state, and over the iniquities that have separated us and God. Such mourning always goes side by side with poverty of spirit.