Why do you ask Me: as a Jew, the man should have known that God shows his standard of goodness in his commandments. The man's questions show he did not trust God's revealed will (also Matthew 19:20).
there is only One who is good: The man might have thought he could be perfectly good on his own, but only God is the truly good. So, God is from where all goodness comes.
keep the commandments: Jesus might be trying to show the man his own sinfulness by asking him to do the impossible. Or more probably, he is restating that good works are the clear measure of how true a person's faith is (see 7:13–27).