Albert Barnes Commentary Mark 10:21

"And Jesus looking upon him loved him, and said unto him, One thing thou lackest: go, sell whatsoever thou hast, and give to the poor, and thou shalt have treasure in heaven: and come, follow me." — Mark 10:21 (ASV)

Jesus—loved him. What occurred afterwards showed that the young man did not love the Savior, or was not a true disciple. So, this expression simply denotes natural affection, or it means that Jesus was pleased with his amiability, his morality, and his external regard for the law of God. At the same time, this was entirely consistent with deep sorrow that he would not give his heart to God, and with deep abhorrence of such a love of the world as to blind the mind to the beauty of true religion, and to lead to the rejection of the Messiah and the destruction of the soul.

One thing thou lackest. When the young man came to Jesus, he asked him, What lack I yet? (Matthew 19:20). This question Mark has omitted, but he has retained the answer. The answer means— There is one thing still lacking. Though all that you have said may be true, still, to make the system complete, or to show that you really are disposed to keep the commands of God, go and sell your property. See whether you love God more than you do your wealth. By doing that, you will show that your love of God is supreme; that your obedience is not merely external and formal, but sincere and real; the thing, now lacking, will be supplied.