A certain Samaritan - The Samaritans were the most inveterate foes of the Jews. They had no dealings with each other. See the notes at Matthew 10:5. It was this fact which rendered the conduct of this good man so striking, and which was thus set in strong contrast with the conduct of the priest and the Levite. "They" would not help their own afflicted, and wounded countryman. "He," who could not be expected to aid a Jew, overcame all the usual hostility between the people; saw in the wounded man a neighbor, a brother, one who needed aid; and kindly denied himself to show kindness to the stranger.

Samaritan is mentioned merely to show that he was a person from whom a Jew had no right to expect any help or relief, because of the enmity which subsisted between the two nations.

But a certain Samaritan,.... By whom Christ may be meant; not that he was really so, for he was a Jew, a son of Abraham, and of David, according to the flesh, but he was so called by the Jews, John 8:48 and was treated as such by them: and since it is the design of the parable in general to show, that he that does acts of kindness and mercy to persons in distress, is a neighbour in the truest sense, though he is not an Israelite, but even a Samaritan, who was, above all men, hated by the Jews; why may it not be thought to be the view of Christ in particular, to prove himself the best neighbour and friend of men, though he was traduced by the Jews as a Samaritan?
as he journeyed; which may design the assumption of human nature, which is sometimes expressed, by his coming from his Father, by his descending from heaven, and coming into this world; which phrases intend his incarnation, and carry in them the idea of journeying: for as his ascent to heaven is expressed by taking a journey, Matthew 25:15 so may his descent from heaven; and while he was here on earth, he was as a stranger and pilgrim, as a sojourner and traveller:
came where he was; put himself in the legal place, and stead of his people, who fell with the rest of mankind in Adam; he became their surety from eternity, and clothed himself with their nature in time; he took upon him their sins, as their representative, and fulfilled the righteousness of the law on their account, and bore the penalty of it in their room:
and when he saw him; Christ saw the elect before the fall, as they were chosen in him, and given unto him, in all the glory they were to be brought into; when he loved them, and his delight was with them: and when he came to redeem them, he saw them as follows; as lost, as weak and strengthless, as wicked, and as the worst of sinners, as ungodly, and enemies, as children of wrath by nature, as others; and he shed his blood for them, and washed them from their sins, that he might present them to himself a glorious church, without spot or wrinkle, or any such thing; just such an one he had seen them to be, in the glass of his Father's purposes, in his council and covenant; and he sees them in their blood, and in the impurity of their nature, when he comes to call them by his grace:
and he had compassion on him. The compassion of Christ on his elect, is to be seen in his eternal covenant engagements; for his tender mercies have been ever of old; and in his assumption of their nature, which was through his own, as well as the tender mercy of his Father; and in the redemption of them, which was in love and pity; and also in their regeneration and conversion, for the great things there, and then done for them by him, are owing to his compassion.

Samaritan--one excommunicated by the Jews, a byword among them, synonymous with heretic and devil (John 8:48; see on Luke 17:18).
had compassion--His best is mentioned first; for "He who gives outward things gives something external to himself, but he who imparts compassion and tears gives him something from his very self" [GREGORY THE GREAT, in TRENCH]. No doubt the priest and Levite had their excuses--It is not safe to be lingering here; besides, he's past recovery; and then, may not suspicion rest upon ourselves? So might the Samaritan have reasoned, but did not [TRENCH]. Nor did he say, He's a Jew, who would have had no dealings with me (John 4:9), and why should I with him?

But a certain Samaritan came where he was - It was admirably well judged to represent the distress on the side of the Jew, and the mercy on that of the Samaritan. For the case being thus proposed, self interest would make the very scribe sensible, how amiable such a conduct was, and would lay him open to our Lord's inference. Had it been put the other way, prejudice might more easily have interposed, before the heart could have been affected.