"The Word of God is oil, because it teaches us to be kind to others with cheerfulness."

15. And that wine may cheer the heart of man. Here he explains for what purpose bread is brought out from the earth. It must not be brought out in vain, just as they should not eat the grass in vain and use the plants in other necessities, but so that they might bring out the bread all the more freely, not revel in idleness. Then, when the bread has been brought out, that wine may cheer the heart of man, that he may make the face cheerful with oil, and that bread may strengthen man’s heart. In these three it is expressed how the Word of God disposes a man first to God, second, to the neighbor, third, to himself. First, the wine is the Word of God that makes the heart of man rejoice in the Lord. He did not say “flesh” of man, but “heart,” whose joy in the conscience God alone sees and man alone knows, as Ps. 32:11 says: “Be glad in the Lord and rejoice, O righteous”; and Ps. 68:3, “Let the righteous be joyful in the Lord, etc.” Thus wine cheers the heart of man, though in the meantime he nevertheless grieves and carries the cross of Christ in his body. For the heart is cheered by this wine, but the flesh is saddened, rather. Second, “that he may make the face cheerful with oil.” The Word of God is oil, because it teaches us to be kind to others with cheerfulness. Hence he aptly says, “that he may make cheerful,” as if to say, “he should be cheerful toward the outside.” Rom. 12:8 reads: “He who shows mercy, with cheerfulness.” “For God loves a cheerful giver” (2 Cor. 9:7). Besides, by saying “face” he means that He may make the outward behavior cheerful with the oil of mercy, which is shared with others. So then, the bread having been brought forth out of the earth, He brings it about that the wine makes glad the heart of man, He brings it about that a man cheerfully bestows the oil of mercy on others, his neighbors in whatever need they may be. Thus he is now properly disposed toward God and the neighbor through the Word of God. So the Samaritan poured in oil and wine for his neighbor, so that we should do likewise (Luke 10:34). Third, “and that bread may strengthen man’s heart.” Man is weak in himself, and therefore he is strengthened by the bread, so that he can rejoice in God and have mercy on his neighbor.

Luther

On Psalm 104:15; LW 11