Catena Aurea on Matthew 6:24: Glossa Ordinaria

Or; He seems to allude to two different kinds of servants; one kind who serve freely for love, another who serve servilely from fear. If then one serve two masters of contrary character from love, it must be that he hate the one; if from fear, while he trembles before the one, he must despise the other. But as the world or God predominate in a man’s heart, he must be drawn contrary ways; for God draws him who serves Him to things above; the earth draws to things beneath; therefore He concludes, “Ye cannot serve God and mammon.”