The synoptic gospels (Matthew, Mark, and Luke) record the occasion where a wealthy young ruler came to Jesus and asked Him, “Good Teacher, what shall I do that I may inherit eternal life?”  Before Jesus answered the question, He asked the young man a question, “Why do you call Me good?”, further stating that, “No one is good but One, that is, God”.

Why did Jesus say, “Why do you call Me good?”  

While it may not be possible to know for sure, some possible reasons can be ruled out, which are discussed below. 

In considering this question it’s helpful to read all three passages in full, because each of the three passages contains information that the others don’t.  Only Matthew reveals that the man was young; only Mark says he knelt before Jesus and that Jesus loved him; and only Luke states that he was a ruler.  Here they are for convenience.

Mat 19:16  Now behold, one came and said to Him, “Good Teacher, what good thing shall I do that I may have eternal life?”
Mat 19:17  So He said to him, “Why do you call Me good? No one is good but One, that is, God. But if you want to enter into life, keep the commandments.”
Mat 19:18  He said to Him, “Which ones?” Jesus said, ” ‘YOU SHALL NOT MURDER,’ ‘YOU SHALL NOT COMMIT ADULTERY,’ ‘YOU SHALL NOT STEAL,’ ‘YOU SHALL NOT BEAR FALSE WITNESS,’
Mat 19:19  ‘HONOR YOUR FATHER AND YOUR MOTHER,’ and, ‘YOU SHALL LOVE YOUR NEIGHBOR AS YOURSELF.’ “
Mat 19:20  The young man said to Him, “All these things I have kept from my youth. What do I still lack?”
Mat 19:21  Jesus said to him, “If you want to be perfect, go, sell what you have and give to the poor, and you will have treasure in heaven; and come, follow Me.”
Mat 19:22  But when the young man heard that saying, he went away sorrowful, for he had great possessions.

Mar 10:17  Now as He was going out on the road, one came running, knelt before Him, and asked Him, “Good Teacher, what shall I do that I may inherit eternal life?”
Mar 10:18  So Jesus said to him, “Why do you call Me good? No one is good but One, that is, God.
Mar 10:19  You know the commandments: ‘DO NOT COMMIT ADULTERY,’ ‘DO NOT MURDER,’ ‘DO NOT STEAL,’ ‘DO NOT BEAR FALSE WITNESS,’ ‘Do not defraud,’ ‘HONOR YOUR FATHER AND YOUR MOTHER.'”
Mar 10:20  And he answered and said to Him, “Teacher, all these things I have kept from my youth.”
Mar 10:21  Then Jesus, looking at him, loved him, and said to him, “One thing you lack: Go your way, sell whatever you have and give to the poor, and you will have treasure in heaven; and come, take up the cross, and follow Me.”
Mar 10:22  But he was sad at this word, and went away sorrowful, for he had great possessions.

Luk 18:18  Now a certain ruler asked Him, saying, “Good Teacher, what shall I do to inherit eternal life?”
Luk 18:19  So Jesus said to him, “Why do you call Me good? No one is good but One, that is, God.
Luk 18:20  You know the commandments: ‘DO NOT COMMIT ADULTERY,’ ‘DO NOT MURDER,’ ‘DO NOT STEAL,’ ‘DO NOT BEAR FALSE WITNESS,’ ‘HONOR YOUR FATHER AND YOUR MOTHER.’ “
Luk 18:21  And he said, “All these things I have kept from my youth.”
Luk 18:22  So when Jesus heard these things, He said to him, “You still lack one thing. Sell all that you have and distribute to the poor, and you will have treasure in heaven; and come, follow Me.”
Luk 18:23  But when he heard this, he became very sorrowful, for he was very rich.

Was the young ruler trying to flatter Jesus by calling him “Good Teacher”?  This is unlikely as the young ruler appeared to be sincere, as firstly he knelt before Jesus, and secondly and more importantly Jesus loved him.

So Jesus wasn’t questioning the young man’s sincerity.

Neither was Jesus telling the young man, “Don’t call me good, because that’s a title for God alone.”  For Jesus was God in the flesh and lived a sinless life and therefore was indeed good.

In asking the young man, “Why do you call Me good? No one is good but One, that is, God”, Jesus was affirming His divinity. 

So then, why did Jesus say, “Why do you call Me good?”   

Jesus often used questions as a means to teach people. Perhaps Jesus was helping the young man to make the right decision.  Jesus no doubt knew what He was going to tell the young man to do (give his possessions to the poor) was going to be very difficult to hear.  If the young knew that what he was about to hear came from the very lips of God, then that might help him make the right decision.  Sadly, the Bible indicates that the young man didn’t follow through on Jesus’s instruction.