And probably most people know the story was presented by Jesus himself. The whole account is given in Luke 10:25-37.
But what was it that prompted Jesus to tell the Story of the Good Samaritan? Why would he give such a detailed account of all the good deeds of the Samaritan in contrast to other seemingly "good" people such as a priest and a Levite?
It was in response to a certain lawyer. What was it about this lawyer that caused Jesus to give the true teaching of the Law? It was because of this:
Luke 10:29 - But he, willing to justify himself, ...
It was because the lawyer wanted to justify himself. How? By his good deeds. How I struggled with this whole account of the Good Samaritan! Jesus did not give the man the Gospel, he gave him the Law! Why Jesus would you do this?
And Jesus answers: Do you too want to justify yourself?
And at the moment that God re-kindled my faith again, I had to answer: No Lord. I can't justify myself, but you have already done it for me on the cross. I know this only because you have justified the whole world apart from anything I have done.
This is what C.F.W. Walther taught - Universal, Objective Justification! (the Gospel)