At this point the young man asks Jesus which commandments should I keep?Mt 19:17 And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.
Matthew 19:18-19 He saith unto him, Which? Jesus said, Thou shalt do no murder, Thou shalt not commit adultery, Thou shalt not steal, Thou shalt not bear false witness, Honour thy father and thy mother: and, Thou shalt love thy neighbour as thyself.
The young man says oh I've kept all those, but what do I LACK?
If keeping the commandments would give the young man eternal life, why then does he ask what he "lacks"?
Matthew 19:21 Jesus said unto him, If thou wilt be perfect, go and sell that thou hast, and give to the poor, and thou shalt have treasure in heaven: and come and follow me.
Here we see what the young man lacked. His heart was not right in that he was not willing to give up his money to help the poor, proving that he had not kept the commandment to love thy neighbor or the commandment to have no other gods before God. His money was his god. He needed the grace of God.