To understand the radical nature of this parable, we must understand who the characters are. Priests were, much as they are today, those who sacrificed to God on his holy altar in the Temple in Jerusalem; these were the mediators between God and men. Levites were from the priestly class but were not priests (they did not offer sacrifices), they were much like was deacons are today: they upheld the Temple and assisted the priests. Samaritans, on the other hand, were far removed. They were part of the Ten Northern Tribes of Israel that broke from Judea, and so from Jerusalem and her Temple, centuries earlier; they were taken by the Assyrians in the 8th Century BC and intermarried, so they were not pure Israelites; they were not even monotheists, they believed in the God of Israel but they believed in other pagan gods all the same. The Jews who belonged to the true faith in the South, where Jerusalem and the Temple was, hated the Samaritans for all these reasons.
Amazingly, Jesus uses one of these hated ones to relay his message of love. Love is, for the Catholic, not simply a feeling, it is an action. Even more, love in the grace of God is a share in the love of God himself, so it is supernatural. This supernatural love is given to us in baptism and increases in us through the sacraments (i.e. the Eucharist, Confession). With faith, our love increases for God and for neighbor, and we are enabled to do things unimaginable. In the Early Church, Christians showed so much compassion on their own people and pagans alike that even pagan leaders were jealous; they tried to force their people to be as charitable as the Christians but could not. They could not because they did not have this great grace; but we do, we are given the gifts to fulfill these great commandments, so that we not only can love our great God, but even our greatest enemy.