Objection 1. It would seem that it was not fitting for the Son of God to assume human nature of the stock of Adam, for the Apostle says (Hebrews 7:26): "For it was fitting that we should have such a high priest... separated from sinners." But He would have been still further separated from sinners had He not assumed human nature of the stock of Adam, a sinner. Hence it seems that He ought not to have assumed human nature of the stock of Adam.
Objection 2. Further, in every genus the principle is nobler than what is from the principle. Hence, if He wished to assume human nature, He ought to have assumed it in Adam himself.
Objection 3. Further, the Gentiles were greater sinners than the Jews, as a gloss says on Galatians 2:15: "For we by nature are Jews, and not of the Gentiles, sinners." Hence, if He wished to assume human nature from sinners, He ought rather to have assumed it from the Gentiles than from the stock of Abraham, who was just.
On the contrary, (Luke 3), the genealogy of our Lord is traced back to Adam.
I answer that, As Augustine says (De Trin. xiii, 18): "God was able to assume human nature elsewhere than from the stock of Adam, who by his sin had fettered the whole human race; yet God judged it better to assume human nature from the vanquished race, and thus to vanquish the enemy of the human race." And this for three reasons: First, because it would seem to belong to justice that he who sinned should make amends; and hence that from the nature which he had corrupted should be assumed that whereby satisfaction was to be made for the whole nature. Secondly, it pertains to man's greater dignity that the conqueror of the devil should spring from the stock conquered by the devil. Thirdly, because God's power is thereby made more manifest, since, from a corrupt and weakened nature, He assumed that which was raised to such might and glory.
Reply to Objection 1. Christ ought to be separated from sinners as regards sin, which He came to overthrow, and not as regards nature which He came to save, and in which "it behooved Him in all things to be made like to His brethren," as the Apostle says (Hebrews 2:17). And in this is His innocence the more wonderful, seeing that though assumed from a mass tainted by sin, His nature was endowed with such purity.
Reply to Objection 2. As was said above (ad 1) it behooved Him Who came to take away sins to be separated from sinners as regards sin, to which Adam was subject, whom Christ "brought out of his sin," as is written (Wisdom 10:2). For it behooved Him Who came to cleanse all, not to need cleansing Himself; just as in every genus of motion the first mover is immovable as regards that motion, and the first to alter is itself unalterable. Hence it was not fitting that He should assume human nature in Adam himself.
Reply to Objection 3. Since Christ ought especially to be separated from sinners as regards sin, and to possess the highest innocence, it was fitting that between the first sinner and Christ some just men should stand midway, in whom certain forecasts of (His) future holiness should shine forth. And hence, even in the people from whom Christ was to be born, God appointed signs of holiness, which began in Abraham, who was the first to receive the promise of Christ, and circumcision, as a sign that the covenant should be kept, as is written (Genesis 17:11).
Source: Summa Theologica (New Advent)