It is one of those Tradition-only teachings to be sure, albeit grounded in reason.
Jesus felt pain, so why not Mary.
Jesus' pain was salvific. Likewise when we experience pain we apply it to our salvation in imitation of Christ. Mary, already saved, had nothing to apply in that sense.
1. Declaring the power of salvific suffering, the Apostle Paul says: "In my flesh I complete what is lacking in Christ's afflictions for the sake of his body, that is, the Church"(1).These words seem to be found at the end of the long road that winds through the suffering which forms part of the history of man and which is illuminated by the Word of God. These words have as it were the value of a final discovery, which is accompanied by joy. For this reason Saint Paul writes: "Now I rejoice in my sufferings for your sake"(2). The joy comes from the discovery of the meaning of suffering, and this discovery, even if it is most personally shared in by Paul of Tarsus who wrote these words, is at the same time valid for others. The Apostle shares his own discovery and rejoices in it because of all those whom it can help—just as it helped him—to understand the salvific meaning of suffering.
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7 [...] the reality of suffering prompts the question about the essence of evil: what is evil?
This questions seems, in a certain sense, inseparable from the theme of suffering. The Christian response to it is different, for example, from the one given by certain cultural and religious traditions which hold that existence is an evil from which one needs to be liberated. Christianity proclaims the essential good of existence and the good of that which exists, acknowledges the goodness of the Creator and proclaims the good of creatures. Man suffers on account of evil, which is a certain lack, limitation or distortion of good. We could say that man suffers because of a good in which he does not share, from which in a certain sense he is cut off, or of which he has deprived himself. He particularly suffers when he a ought"—in the normal order of things—to have a share in this good and does not have it.
Thus, in the Christian view, the reality of suffering is explained through evil, which always, in some way, refers to a good.
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11. [...] The Book of Job is not the last word on this subject in Revelation. In a certain way it is a foretelling of the Passion of Christ. But already in itself it is sufficient argument why the answer to the question about the meaning of suffering is not to be unreservedly linked to the moral order, based on justice alone.
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19. [...] And the Apostle Paul in the Letter to the Galatians will say: "He gave himself for our sins to deliver us from the present evil age"(56), and in the First Letter to the Corinthians: "You were bought with a price. So glorify God in your body "(57).
With these and similar words the witnesses of the New Covenant speak of the greatness of the Redemption, accomplished through the suffering of Christ. The Redeemer suffered in place of man and for man. Every man has his own share in the Redemption. Each one is also called to share in that suffering through which the Redemption was accomplished.
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24 [...] In the Paschal Mystery Christ began the union with man in the community of the Church. The mystery of the Church is expressed in this: that already in the act of Baptism, which brings about a configuration with Christ, and then through his Sacrifice—sacramentally through the Eucharist—the Church is continually being built up spiritually as the Body of Christ. In this Body, Christ wishes to be united with every individual, and in a special way he is united with those who suffer. The words quoted above from the Letter to the Colossians bear witness to the exceptional nature of this union. For, whoever suffers in union with Christ— just as the Apostle Paul bears his "tribulations" in union with Christ— not only receives from Christ that strength already referred to but also "completes" by his suffering "what is lacking in Christ's afflictions". This evangelical outlook especially highlights the truth concerning the creative character of suffering. [...]
25. The witnesses of the Cross and Resurrection of Christ have handed on to the Church and to mankind a specific Gospel of suffering. The Redeemer himself wrote this Gospel, above all by his own suffering accepted in love, so that man "should not perish but have eternal life"(80). This suffering, together with the living word of his teaching, became a rich source for all those who shared in Jesus' sufferings among the first generation of his disciples and confessors and among those who have come after them down the centuries.
It is especially consoling to note—and also accurate in accordance with the Gospel and history—that at the side of Christ, in the first and most exalted place, there is always his Mother through the exemplary testimony that she bears by her whole life to this particular Gospel of suffering. In her, the many and intense sufferings were amassed in such an interconnected way that they were not only a proof of her unshakeable faith but also a contribution to the redemption of all. In reality, from the time of her secret conversation with the angel, she began to see in her mission as a mother her "destiny" to share, in a singular and unrepeatable way, in the very mission of her Son. And she very soon received a confirmation of this in the events that accompanied the birth of Jesus in Bethlehem, and in the solemn words of the aged Simeon, when he spoke of a sharp sword that would pierce her heart. Yet a further confirmation was in the anxieties and privations of the hurried flight into Egypt, caused by the cruel decision of Herod.
And again, after the events of her Son's hidden and public life, events which she must have shared with acute sensitivity, it was on Calvary that Mary's suffering, beside the suffering of Jesus, reached an intensity which can hardly be imagined from a human point of view but which was mysterious and supernaturally fruitful for the redemption of the world. Her ascent of Calvary and her standing at the foot of the Cross together with the Beloved Disciple were a special sort of sharing in the redeeming death of her Son. And the words which she heard from his lips were a kind of solemn handing-over of this Gospel of suffering so that it could be proclaimed to the whole community of believers.
As a witness to her Son's Passion by her presence, and as a sharer in it by her compassion, Mary offered a unique contribution to the Gospel of suffering, by embodying in anticipation the expression of Saint Paul which was quoted at the beginning. She truly has a special title to be able to claim that she "completes in her flesh"—as already in her heart—"what is lacking in Christ's afflictions ".
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26. [...] the Divine Redeemer wishes to penetrate the soul of every sufferer through the heart of his holy Mother, the first and the most exalted of all the redeemed. As though by a continuation of that motherhood which by the power of the Holy Spirit had given him life, the dying Christ conferred upon the ever Virgin Mary a new kind of motherhood—spiritual and universal—towards all human beings, so that every individual, during the pilgrimage of faith, might remain, together with her, closely united to him unto the Cross, and so that every form of suffering, given fresh life by the power of this Cross, should become no longer the weakness of man but the power of God.
What are we to conclude regarding Mary's birthing pains? The Holy Father makes the distinction, it seems, between the suffering we experience and apply to the redemptive work of Christ for our own salvation, and the suffering of Christ and His Blessed Mother done on behalf of others. Our Lady did not suffer physical pain because her final justification had already occurred at her conception. She, uniquely, had nothing to complete in her redeemed flesh. Her profound sorrow is all mental as she is carrying the Gospel of her Son's suffering to us through her motherhood. Her birthing pain is happening at Bethlehem, but rather at Golgotha.
Our Lady of the Seven Sorrows
Adriaen Isenbrant