5 And in many other places also Christ is clearly doing this; He does not openly convict, but by His answer He manifests the purpose of them that are coming unto Him. Thus to him again that said, “Good Master,” and had thought by such flattery to gain His favor, according to his purpose He made answer, saying, “Why do you call me good? There is none good but one, that is, God.”
And when they said unto Him, “Behold, Your mother and Your brethren seek You;” forasmuch as these were under the influence of some human infirmity, not desiring to hear something profitable, but to make a display of their relationship to Him, and therein to be vainglorious; hear what He says: “Who is my mother, and who are my brethren?”
And again to His brethren themselves, saying unto Him, “Show yourself to the world,” and wishing thence to feed their vainglory, He said, “Your time” (so He speaks) “is always ready, but my time is not yet come.”
And in the opposite cases too He does so; as in that of Nathanael, saying, “Behold an Israelite indeed, in whom is no guile.” And again, “Go and show John again those things which you do hear and see.” For neither in this did He reply to the words, but to the intention of him that sent them. And with the people again in like manner, He addresses His discourse unto their conscience, saying, “What went ye out into the wilderness to see?” That is because they were probably feeling about John, as though he had been a sort of easy and wavering person; to correct this their suspicion, He says, “What went ye out into the wilderness to see? A reed shaken with the wind?” or, “a man clothed with soft raiment?” by both these figures declaring, that he was neither of himself a waverer, nor would be softened by any luxury. Thus then in the present case also He makes His answer to their meaning.
And see how in this also He shows forth great moderation: in that He said not, “I have it indeed, but despise it,” but “I have it not.” Do you see what exact care goes along with His condescension? Even as when He eats and drinks, when He seems to be acting in an opposite way to John, this too He does for the sake of the Jews' salvation, or rather for that of the whole world, at once both stopping the mouths of the heretics, and desiring to win also more abundantly those of that day to Himself.
Source: Homilies on the Gospel of St. Matthew (New Advent)