Their conversation immediately follows Jesus' statement: "Let the little children come to Me, and do not forbid them; for of such is the kingdom of heaven."
The rich ruler had a lot to offer. He had power, wealth, influence. He was ready to do some great work of benevolence or service. Maybe he thought Jesus would ask him to fast or build or feed the poor- some great work which HE was capable of doing. But the Lord told him he had to realize that Jesus was the source of what he wanted- it was not within his own power. Jesus wanted him to become like the little children he had just blessed: insignificant and totally dependant.
What held him back was the way he looked at what was "his." He grasped tightly his security, for fear of. . . probably the same things we all fear. He had a trust issue. He needed NO FEAR!
Jesus said, in essence, surrender or go home.
Fear asks, will I be alright? Jesus answers, More than alright.