16 Let us see, then, how the questioner styled Him, beside calling Him good. He said, Good Master, what good thing shall I do? Adding to the title of “good” that of master. If Christ then did not chide because He was called good, it must have been because He was called “good Master.” Further the manner of His reproof shows that it was the disbelief of the questioner, rather than the name of master, or of good, which He resented. A youth, who provides himself upon the observance of the law, but did not know the end of the law, which is Christ, who thought himself justified by works, without perceiving that Christ came to the lost sheep of the house of Israel, and to those who believe that the law cannot save through the faith of justification, questioned the Lord of the law, the Only-begotten God, as though He were a teacher of the common precepts and the writings of the law.
But the Lord, abhorring this declaration of irreverent unbelief, which addresses Him as a teacher of the law, answered, Why do you call Me good? and to show how we may know, and call Him good, He added, None is good, save one, God, not repudiating the name of good, if it be given to Him as God.
Source: On the Trinity (New Advent)