The prestigious Oxford Bible Commentary on the Gospels (published by Oxford University Press) is clear that the author of Matthew’s gospel deliberately changed the words of Jesus in the earlier gospel of Mark because they were “embarrassing” to him.

Why were they embarrassing?

Here is the story in the earliest gospel to be written, the gospel of Mark:

As he was setting out on a journey, a man ran up and knelt before him, and asked him, “Good Teacher, what must I do to inherit eternal life?” Jesus said to him, “Why do you call me good?No one is good but God alone.”

The Oxford Bible Commentary on the Gospels comments (on page 113):

The evident embarrassment caused to later Christians (Matthew!) by the story in which Jesus appears implicitly to reject the notion that he himself is ‘good’ [in Mark 10:17] suggests that we have here a…