Why did the Son become incarnate? That is a good question. Several people on the A-Team (Athanasius, Augustine, Aquinas and Anselm) have all taken a shot at answering this question. Usually the answer gets tied in to the doctrine of atonement. Here is what Athanasius has to say about that question:
For speaking of the manifestation of the Savior to us it is necessary also to speak of the origin of human beings, in order that you might know that our own cause was the occasion of his descent and that our own transgression evoked the Word’s love for human beings, so that the Lord both came to us and appeared among human beings. For we were the purpose of his embodiment, and for our salvation he so loved human beings as to come and appear in a human body. Thus, then, God created the human being and willed that he should abide in incorruptibility; but when humans despised and overturned the comprehension of God, devising and contriving evil for themselves, as we said in the first work (Against the Gentiles), then they received the previously threatened condemnation of death, and thereafter no longer remained as they had been created, but were corrupted as they had contrived; and, seizing them, death reigned. (De Incarnationae S.4)
So why did the Son become incarnate? Quite simply, the Son became incarnate to save us from the death we had brought upon ourselves.
Thanks be to God!