The Synoptic Gospels must reconcile stories of each other. For example, the story of the rich young ruler (of which only in Matthew is he said to be young and only in Luke is he said to be a ruler. Matt 19:20 and Luke 18:18).

When comapared to Luke 18:18-19, we see that Luke agrees with Mark nearly word for word (not including the introduction of the scene). Mark, rather than Matthew, might have been Luke's source, because the only reason for thinking that any of these Gospels is the source for the others is their verbal agreement. But was Matthew the source for Mark or was Mark the source for Matthew?

In this passage, the man who approaches Jesus uses the term "good" in both accounts, but in Matthew he uses it to refer to the deed he must do, whereas in Mark he uses it to refer to Jesus. Therefore, the dialogue in Mark makes sense: Jesus wants to know why the man has called him good when only God is good. But the flow of the dialogue in Matthew seems a bit strange: why would Jesus object to the man asking him about what is good, on the grounds that God alone is good?

To understand the story, consider that Matthew's dialogue has become convoluted because he has shifted the adjective away from Jesus and onto the deed that must be done. By doing so, he interrupted the flow of the conversation. What would have compelled him to make the change? One possibility is that there was something in Mark's story that proved disturbing to him. On closer examination one may be able to detect what it was. One way to read Mark's account is to think that Jesus is claiming not be good ("Why do you call me good? There is no one good but God!") If Matthew realized that Jesus' words might be taken this way, he may have changed the account slightly simply by moving the adjective "good."

(Adopted from Bart D. Ehrman's The New Testament: A Historical Introduction to the Early Christian Writings, Fourth Edition. New York: Oxford University Press, 2008.)

See Peculiarities Of The Gospels