2 A passage, therefore, of a moral character is here inserted. The good Teacher does what He thereby reminds us ought to be done, and by His own example instructed His disciples that care for their parents ought to be a matter of concern to pious children: as if that tree to which the members of the dying One were affixed were the very chair of office from which the Master was imparting instruction. From this wholesome doctrine it was that the Apostle Paul had learned what he taught in turn, when he said, “But if any provide not for his own, and especially for those of his own house, he has denied the faith, and is worse than an infidel.” And what are so much home concerns to any one, as parents to children, or children to parents?
Of this most wholesome precept, therefore, the very Master of the saints set the example from Himself, when, not as God for the hand-maid whom He had created and governed, but as a man for the mother, of whom He had been created, and whom He was now leaving behind, He provided in some measure another son in place of Himself. And why He did so, He indicates in the words that follow: for the evangelist says, “And from that hour the disciple took her unto his own,” speaking of himself.
In this way, indeed, he usually refers to himself as the disciple whom Jesus loved: who certainly loved them all, but him beyond the others, and with a closer familiarity, so that He even made him lean upon His bosom at supper; in order, I believe, in this way to commend the more highly the divine excellence of this very gospel, which He was thereafter to preach through his instrumentality.
Source: Tractates on the Gospel of John (New Advent)