If we look at the parable, it is clear that the cynic speaking to Jesus wants to know who his neighbor is. The Jews had taken the technical meaning of the word, which often can be translated in the Old Testament as friend, and applied it only to those they like within the covenant community. This is made evident throughout the Gospels of both Luke and Matthew. They have nothing to do with Gentiles, sinners, prostitutes, Samaritans, etc., even though all are within the visible covenant community. Hence, this teacher likely thinks he only has obligations to his friends, since he was seeking to justify himself (i.e., seeking to affirm that he already does this).
Instead, Jesus flips the script, in accordance with Luke's larger argument, that he should be more concerned about identifying himself as a true covenant member. Hence, Jesus makes the argument that anyone in the covenant that evidences his love for the fellow covenant member evidences that he himself is a covenant member. That is the entire point of Luke's particular emphasis in his Gospel.
But notice, once again, that there is every indication that everyone is not the neighbor. Jesus actually asks, "Who became the neighbor of the injured man?" Notice, that if you become a neighbor, that implies that you were not one before. If everyone is a neighbor, then how does one become a neighbor? One can say, he became a human being; but if he already is a human being, this means that he simply does something that evidences his identity.
It asks, "Who became the neighbor?" This implies that one must actually do something to become the neighbor. One is not intrinsically a neighbor. This means that everyone is not a neighbor, which is why the answer to Jesus' question is not, "Everyone." Perhaps, one can argue that everyone can become a neighbor, regardless of their covenant status, but again, this would ignore Luke's point, which is all about doing the works toward covenant members that show forth covenant status.
It further tells us that if we think the Samaritan is a pagan, rather than a believer/covenant member, then we end up making this parable about becoming a covenant member via a good work. If we argue that he is already a covenant member, and became, in practice, what he claimed in identity, then we have a teaching consistent with all of Jesus' other teaching and the teaching of the Gospel itself. Otherwise, we must conclude that pagans become covenant members by virtue of their helping covenant members (i.e., doing good works toward covenant members), since that is who the injured man is in the story.
In other words, no matter how one slices it, this is not a generic story about doing good to one's fellow man, regardless of his covenant status. That sort of thinking just flips what is said on its head. The person in need in the story is a covenant member. He is a Jew travelling on the road between Jericho and Jerusalem.
He is also ultimately Christ. The Good Samaritan is the One rejected by the Jews, but really is the true neighbor to the true covenant member. He dies for His people. If this imagery is true, as I do think Luke seems at least to hint at it, then everyone is not the recipient of His kingdom blessing of the cross either. He dies only for His elect. He applies that sacrifice only to His elect. According to the neo-Marcionites, however, this makes Christ mean, and by their own warped definition, not a neighbor.
In the end, the story is about proving one's own covenant status by how he treats others in the covenant who are disadvantaged/poor/marginalized. To make it about people in general is simply to add elements to the story that stem from an inclusivistic theology that Luke does not share.