Jesus does not immediately address the question posed to him. Why not? It seems he wants the questioner to think, to make him wonder to whom he is addressing this question, and to realize what it means to address Jesus as Good Teacher.
Jesus points out that there is no one good except God.
Yes, God is good. He surely knew that. Praise the Lord, for he is good; for his steadfast love endures forever
(see Psalm 118:1; Ezra 3:11; Jeremiah 33:11). By pointing him to God that is good, Jesus wants to make it clear to him who is standing before him. Does he realize that?
Do we realize who Jesus is? That we humans have to deal with God in Jesus? That he is more than a human, and that he can therefore speak with divine authority. That he can therefore also say, Follow me!
(Luke 18:22). Jesus' words have a deeper layer of meaning. That had to really sink in with this ruler. The same applies to each of us.
19 Jésus lui répondit: Pourquoi m'appelles-tu bon? Il n'y a de bon que Dieu seul.