So Jesus said to him, "Why do you call me good? No one is good but One, that is, God.
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.'"
And he said, "All these things I have kept from my youth."
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."
But when he heard this, he became very sorrowful, for he was very rich."
Luke 18:18-23 (NKJV)
A necessity is something that you really cannot live without. If you asked people what things were a necessity for them to live a happy and productive life you would have a lot of different answers, depending on who you asked. Most people would agree that food, water, clothing, and shelter are basic necessities. But, beyond that, what people might consider necessities can vary. There are many things people may consider necessary to their life that in reality are not necessary.
When Jesus met the rich young ruler, this man wanted to know what was necessary to inherit eternal life. In response, Jesus listed several of the Ten Commandments. The rich ruler told Jesus how he had kept these commandments, in his mind he had done what was necessary. Jesus then told the ruler to sell his possessions and follow Him. This brought a sad reaction from the rich ruler, unfortunately he considered his riches a necessity and this hindered him from following Jesus.
The problem was that this ruler did not know what was really a necessity, he did not want to let go of what he had to go and follow Jesus. Many people talk about being a follower of Jesus, but they are unwilling to leave behind some things that they consider necessities, which are actually hindering their relationship with God and the things He wants them to do. We all need to be willing to let go of anything God talks to us about, He knows best what is really necessary for us to have to follow His plan for our life.