In Luke chapter ten, we are given the account of Christ's encounter with a lawyer, who asks the Lord what to do to "inherit Eternal Life":
Luke 10:25-29
25 And, behold, a certain lawyer stood up, and tempted him, saying, Master, what shall I do to inherit eternal life?
26 He said unto him, What is written in the law? how readest thou?
27 And he answering said, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy strength, and with all thy mind; and thy neighbour as thyself.
28 And he said unto him, Thou hast answered right: this do, and thou shalt live.
29 But he, willing to justify himself, said unto Jesus, And who is my neighbour?
Christ responds by asking him what's written in the Law. The man responds well, stating the two Great Commandments (Matthew 22:37-40, Mark 12:29-31). Christ affirms his response, and tells him that if he keeps them, he will live.
Firstly, note how similar this is to Christ's encounter with the rich young ruler (Matthew 19:16-26, Mark 10:17-27, Luke 18:18-27), which is covered in this article. Both ask more or less the same question, although it's easier to detect the rich young ruler's initial misconception by what he says - asking Christ "what good thing shall I do, that I may have eternal life" (Matthew 19:16)".
The lawyer in the passage is said to be "tempting" Christ (Luke 10:25), which usually indicates asking from a dishonest foundation (Matthew 16:1, 19:3, 22:18, Mark 8:11, 10:2, 12:15, Luke 11:16, 20:23, John 8:6), though sometimes that term is used without a negative interaction taking place (Matthew 22:35, Mark 12:28-34). We do learn from Luke 10:29 that the lawyer was "willing" (wanting) "to justify himself", which does seem to indicate that he had the same misgivings as the rich young ruler.
In any case, Christ does the same thing that He did with the rich young ruler, throwing the Law at him. It is a genuine answer to the question - as there are two ways to get to Heaven. If one keeps the Law, they will be justified by the Law (Romans 2:13), and get to Heaven on their own merit.
However, this creates an impasse for all of mankind, because "all have sinned" (Romans 3:23) - that is, all have violated the Law, since "sin is the transgression of the Law" (1 John 3:4). Therefore, the Law only exists to reveal how short mankind falls of God's perfect standard, leaving their only recourse to put their faith in the Savior that God provided to rescue them from the condemnation of the Law:
Romans 3:19-20, 28
19 Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God.
20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
28 Therefore we conclude that a man is justified by faith without the deeds of the law.
Above, we see the purpose of the Law, and the point of Jesus putting the Law to the lawyer in the passage in question - to give "the knowledge of sin", and to be "our schoolmaster to bring us unto Christ, that we might be justified by faith" (Galatians 3:24). Christ has given him an impossible commandment, to the purpose of showing him that he isn't good enough to get to Heaven.
The lawyer, if he were honest with himself, would find this response to be horrible news, as his own conscience bears witness that he hasn't always kept the two Great Commandments. This plays out in the passage, leading him to ask a follow-up question, to which Christ responds with the parable of the good Samaritan:
Luke 10:29-37
29 But he, willing to justify himself, said unto Jesus, And who is my neighbour?
30 And Jesus answering said, A certain man went down from Jerusalem to Jericho, and fell among thieves, which stripped him of his raiment, and wounded him, and departed, leaving him half dead.
31 And by chance there came down a certain priest that way: and when he saw him, he passed by on the other side.
32 And likewise a Levite, when he was at the place, came and looked on him, and passed by on the other side.
33 But a certain Samaritan, as he journeyed, came where he was: and when he saw him, he had compassion on him,
34 And went to him, and bound up his wounds, pouring in oil and wine, and set him on his own beast, and brought him to an inn, and took care of him.
35 And on the morrow when he departed, he took out two pence, and gave them to the host, and said unto him, Take care of him; and whatsoever thou spendest more, when I come again, I will repay thee.
36 Which now of these three, thinkest thou, was neighbour unto him that fell among the thieves?
37 And he said, He that shewed mercy on him. Then said Jesus unto him, Go, and do thou likewise.
With his question, the lawyer was hoping to narrow the scope of who his "neighbor" was, in order to lessen the strength of the commandment. The Lord's parable demonstrates that being someone's "neighbor" is not just a classification, based on geography or ethnicity, but a responsibility to all those who someone may encounter, regardless of other factors.
So, with this response, Christ refuses to allow the Law's standard to be diminished, and leaves the man to go his way, knowing that his conscience needs to do more work on him before he is ready to accept his own inability - just as we saw at the conclusion of the encounter with the rich young ruler - the need to understand the "impossibility" (Matthew 19:25-26, Mark 10:26-27, Luke 18:26-27) of self-justification.
The Lord in His preaching repeatedly used the Law for its intended purpose - to convict mankind. Those who would use this passage to teach that someone gets to Heaven by keeping the commandments, are rejecting the lesson that the "schoolmaster" was given to teach them.
The perfect commandment is designed to show shortcomings, and can only be thought to be a method to get to Heaven by those who are dishonest with themselves, or oblivious to how dreadfully high that standard is.