Let us go right to the apex of the prologue of John's Gospel, which is read at the third Mass on Christmas day.
In the Credo there is a line that on this day we recite on our knees: "For us men and for our salvation he came down from heaven." This is the fundamental and perennially valid answer to the question -- "Why did the word become flesh?" -- but it needs to be understood and integrated.
The question put another way is in fact: "Why did he become man 'for our salvation?'" Only because we had sinned and needed to be saved?
There is a vein of the theology inaugurated by Blessed Duns Scotus, a Franciscan theologian, which loosens a too exclusive connection to man's sin and regards God's glory as the primary reason for the Incarnation. "God decreed the incarnation of his Son in order to have someone outside of him who loved him in the highest way, in a way worthy of God."
This answer, though beautiful, is still not the definitive one. For the Bible the most important thing is not, as it was for Greek philosophers, that God be loved, but that God "loves" and loved first (cf. 1 John 4:10, 19). God willed the incarnation of the Son not so much as to have someone outside the Trinity that would love him worthily as to have someone to love in a way worthy of him, that is, to love without measure!
At Christmas, when the child Jesus is born, God the Father has someone to love in an infinite way because Jesus is together man and God. But not only Jesus, but us together with him. We are included in this love, having become members of the body of Christ, "sons in the Son." John's prologue reminds of this: "To those who welcomed him he gave the power to become sons of God."
Therefore, Christ did descend from heaven "for our salvation," but what moved him to come down for our salvation was love, nothing else but love.
Fr. Cantalamessa's conclusion is true and not in dispute. And his proffered correction of Duns Scotus is indeed an improvement on the great theologian's explanation, which in turn had been offered, rightly or wrongly, as an improvement on Thomas Aquinas. The preacher's improvement is this: "God willed the incarnation of the Son not so much as to have someone outside the Trinity that would love him worthily as to have someone to love in a way worthy of him, that is, to love without measure." As we adore the Christ Child, we can surely see Love without measure that is loveable without measure. But there is a problem nonetheless.
Do not the persons comprising the triune God, who are each the same God, "love without measure" just by eternally loving each other? If not, it follows that their mutual love is not infinite and complete and thus does not constitute Love Itself. But "God is Love" (1 John 4) as well as infinite and absolutely perfect. So the conclusion that I've said follows is inadmissible in itself. But if the triune God as such is already love without measure, then what is illuminated by saying that God willed the Incarnation "so as to have someone to love in a way worthy of him?" God already has someone to love that way just by being triune—and necessarily so. It seems we have a dilemma here. The purpose of the Incarnation was, on this account, to do something that God in himself already does completely and necessarily ad intra.
An obvious way to try to resolve the dilemma would be to say that God willed the Incarnation not so as to have someone to love infinitely when he otherwise wouldn't have, but so as to love someone infinitely ad extra as well as ad intra. That makes the love embodied in the Incarnation intelligible, but not necessary. That is, given that God in himself is infinite love necessarily, it makes sense that he also willed the Incarnation in order to manifest such love, even though he didn't "have to" in any meaningful sense. Gratuitous love, after all, is at least as perfect as necessitated love. This would be Mystery in a positive sense: not what is opaque to all knowledge, but what is intelligible without in any way being necessitated.
But a problem remains. If Aquinas was right and God became man only to save us from our sins, then the Incarnation makes more sense than it does on the above account. For it would have made less sense, from the standpoint of infinite love, for God to create our first parents in a state of original justice and holiness but then decline to save them and us from the consequences of their Fall, than it would have for God not to have chosen to become incarnate if man had not sinned in the first place.
Wittgenstein said, for the wrong reasons, that "that of which one cannot speak, thereof one must remain silent." Here we contemplate a Mystery of which it is probably wise not to speak further. The account one prefers depends largely on what one believes the nature of mystery to be. On either the Thomistic account or the more Scotistic one offered by Fr. Cantalamessa, we have "positive" mystery, since even on the Thomistic account, the creation and original elevation of man did not have to occur. But the latter account still makes the mystery of the Incarnation more intelligible than the former. As far as I can tell, there is nothing in the deposit of faith to require adherence to one account of the Great Mystery over the other. And so there is little point in pressing speculation any further. Take your pick, but remember that it cannot go beyond opinion—in this life.