Most assuredly, I say to you that you will weep and lament, but the world will rejoice; and you will be sorrowful, but your sorrow will be turned into joy. A woman, when she is in labour, has sorrow because her hour has come; but as soon as she has given birth to the child, she no longer remembers the anguish, for joy that a human being has been born into the world. Therefore you now have sorrow; but I will see you again and your heart will rejoice, and your joy no one will take from you. (Jn 16: 20-22)
Verse 20: “Most assuredly, I say to you that you will weep and lament, but the world will rejoice;”
Because by reason of their (the disciples) not desiring His death, they quickly ran into the belief that He would not die, and then when they heard that He would die, cast about, not knowing what that “little” meant, He says, “You shall weep and lament.”
“but your sorrow will be turned into joy.”
Then having shown that after grief comes joy, and that grief brings forth joy, and that grief is short, but the pleasure endless, He passes to a common example; and what says He?
Verse 21: “A woman, when she is in labour, has sorrow”
And He has used a comparison which the Prophets also use continually, likening despondencies to the exceeding pains of childbirth. But what He says is of this kind: “Labour pains shall lay hold on you, but the pang of childbirth is the cause of joy”; both confirming His words relative to the Resurrection, and showing that the departing hence is like passing from the womb into the light of day.
As though He had said, “Marvel not that I bring you to your advantage through such sorrow, since even a mother to become a mother, passes in like manner through pain.” Here also He implies something mystical, that He has loosened the labour pangs of death, and caused a new man to be born of them, And He said not, that the pain shall pass away only, but, “she does not even remember it,” so great is the joy which succeeds; so also shall it be with the Saints.
And yet the woman does not rejoice because “a man has come into the world,” but because a son has been born to her; since, had this been the case, nothing would have stopped the barren from rejoicing over another who bears a child. Why then did He say this? Because He introduced this example for this purpose only, to show that sorrow is for a season, but joy lasting: and to show that (death) is a translation unto life; and to show the great profit of their pangs.
He said not, “a child has been born,” but, “A man.” For to my mind He here allludes to His own Resurrection, and that He should be born not unto that death which bore the birth-pang, but unto the Kingdom. Therefore He said not, “a child has been born unto her,” but, “A man has been born into the world.”
[St. John Chrysostom, Homily 79 on the Gospel of St. John]