1) The first thing we need to understand is that the rich young ruler was NOT a regular follower of Jesus. This is seen in Christ's order to follow him in Mark 10:21. This man was not around Jesus long enough to be aware of his divine nature. In his eyes, Jesus was just a human being. If Jesus were to accept his saying that he was indeed good, then this individual would walk around with a false conclusion that mankind is righteous or can become righteous which would go against Matthew 7:11 where Jesus says that all men are evil. So Jesus answered him in light of this person's beliefs about him: That he was nothing more or less than a human being. He helped prevent him from false doctrine about the nature of man.

2) Another thing we have to see is that Jesus taught the people that he was indeed good. He called himself the good shepherd in John 10:11 and he also told us that he ALWAYS does the things that are pleasing to the father in John 8:29. The question Jesus asked him ( Why do you call me good) was a rhetorical question. It was to lead the man towards Christ's true identity. In other words, the point Jesus was trying to make was the following:

If no one is good but God, and you indeed call me good, then why don't you call me God?