There was a barrier keeping the young man in Mark 10:17-31 out of the kingdom: his love of money. Money represented his pride of accomplishment and self-effort. Ironically, his attitude made him unable to keep the first commandment, to let nothing be more important than God. This man came to Jesus wondering what he could do; he left seeing what he was unable to do.
Jesus said it was very difficult for the rich to get into the kingdom of God because the rich have most of their basic physical needs met and thus often become self-reliant. When they feel empty, they can buy something new to dull the pain that was meant to drive them toward God. Their abundance becomes their deficiency. Jesus explained that in the world to come, the values of this world will be reversed.
Commentary from The One Year NIV Devotional NT, April 20, Tyndale House Publishers. 2003.