Teachers could be addressed respectfully, but Jesus refuses to let the adjective 'good' pass as casual flattery. He redirects the man's attention to God's unique goodness.
The exchange occurs before the man demonstrates that he does not yet grasp the radical authority of the person he has approached.
The Old Testament repeatedly celebrates the LORD as good and as the ultimate standard and source of goodness. God's goodness is a foundational confession of Israel's worship.
Jesus asks two linked questions: why the man calls him good, and whether he understands that God alone is good. He does not say, 'I am not good,' nor does he explicitly say, 'Therefore I am God.'
The question exposes the inadequacy of superficial address and presses the man toward deeper reflection.
This passage should not be used as a stand-alone proof either for or against Jesus' deity. It is therefore Supporting.
Within Mark, readers already possess information about Jesus that the rich man lacks. The saying can appropriately be presented as a question about whether the man understands the implications of his own address.
The man continues the conversation but ultimately goes away sorrowful when Jesus commands radical discipleship.