"What good thing shall I do, that I may have eternal life?" the rich man asks our Lord. Now wouldn't it be grand if there were such a thing that we could do, and having done it, we would be assured of eternal salvation? Jesus, though, provides no such easy answer, but He simply states the obvious: keep the commandments. This, the rich man assures Him, he has been doing since his youth (at least externally). This is certainly not the answer he was hoping for, and no doubt he is feeling somewhat impatient. He feels deep down that something is missing, despite his outwardly pious life as an observing Jew. Truly, as St. Augustine said, "Our hearts are ever restless till they find their rest in Thee."
When the man persists, demanding to know the one good thing he still lacks, Jesus throws him a loop: he tells the man to do the one thing he is not prepared to do, the one thing that ever remains an obstacle in his quest for eternal life. He must sell all that he has, give the proceeds to the poor, and then come follow Christ. Thus it is that the rich man shakes his head sadly and turns away.
The truth is, salvation isn't a pleasant bonus we receive as a reward for living a "good life" here on earth--it is rather the whole point of our existense, the reason God called us out of nothingness into being. It is not a part time venture--it is the sole purpose of everything we say, do and think. There simply is no free passport to heaven--nothing less than a total commitment to God is required if we desire eternal life.
Of course we all fall far short of the perfection the Gospel demands--"Be ye perfect, even as your Father in heaven is perfect"--so what then? Shall we, like the rich man, turn sadly away? God forbid! Salvation is not something we achieve in a moment, by doing a certain one good thing. It is, rather, a lifelong process. Speaking realistically, we shall never achieve complete perfection in this fallen world--but it is and ever remains a goal we must strive for until we take our dying breath. And though we will surely miss the mark time and again, genuine and heartfelt repentance is all that's required to restore us to God's grace.