The young man was not yet capable of seeing or believing that to love God is eternal life. How many Christians are?  How many care that they are not? The Lord tells him he must keep the commandments, and when the young man asks, “Which?’ specifies only those that have to do with his neighbor, ending with the highest and most difficult. If I am told, ‘But no man can perfectly keep a single commandment of the second table any more than of the first,’ I respond, surely not—else why should they have been given? Is there no meaning in the word keep except it be qualified by perfectly? That no keeping but a perfect one will satisfy God, I hold with all my heart; but that there is none else he cares for, is one of the lies of the enemy. What father is not pleased with the first tottering attempt of his little one to walk? What father would be satisfied with anything but the manly step of the full-grown son?

The youth responds at once that he has observed the commandments; there must be a keeping of them which, although anything but perfect, is yet acceptable to the heart of him from whom nothing is hid. The immediate end of the commandments never was that men should succeed in obeying them, but that, finding they could not do that which yet must be done, they should be driven to the source of life and law. This had been wrought in the youth; he desired eternal life, of which there was no word in the law: the keeping of the law had served to develop a hunger which no keeping of the law could fill.
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Commentary

by James House

"And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent."
- John 17:3

"If any man will do his will, he shall know of the doctrine, whether it be of God, or whether I speak of myself."
- John 7:17

"Blessed are they which do hunger and thirst after righteousness: for they shall be filled." - Matthew 5:6

The young rich man desired to do a grand something in order to inherit eternal life.  Jesus told him what would bring eternal life: knowing God - by loving and obeying Him more than loving mammon.  The rich man was not ready to obey, because he had not yet grown in true love.

We come to love our neighbors by serving them as Christ would serve them.  The more we serve them, the greater our love for them will grow.   In turn, our obedience in serving our neighbors will cause our knowledge (and love!) of God to grow.  George MacDonald reminds us: "obedience is the soul of knowledge".

But some neighbors are hard to love!  And some act as if they do not want to be loved!  We must start by loving whom we can, and then our capacity to love will be increased.  "Men cannot be righteous without love;  to love a righteous man is the best, the only way to learn righteousness:  the Lord gives us himself to love, and promises his closest friendship to them that overcome." - George MacDonald

Our desire to have eternal life - to know God - will be met when our hunger to be righteous - to do God's will - results in continuous actions of obedience.