Ambrose of Milan

Boast not thyself then that thou hast been a good servant. Thou hast done what thou oughtest to have done. The sun obeys, the moon submits herself, the angels are subject; let us not then seek praise from ourselves. Therefore He adds in conclusion, So likewise ye, when ye have done all good things, say, We are unprofitable servants, we have done that which it was our duty to do.

Catena Aurea by Aquinas ↗

Bede

Does he give thanks to that servant because he did what was commanded of him? I think not. So also you, when you have done all things that are commanded you, say: We are unprofitable servants. If a man, he says, demands not uniform but multiple service from a human servant, and yet does not give him thanks, how much more you, who can do nothing without me? You ought not to measure the merits of labors by the length of time, but by love and voluntary service, always increasing the former by new endeavors. …

On the Gospel of Luke ↗

Bonaventure

Second, as to the necessity of serving, which he takes up, he adds: Does he have gratitude toward that servant because he did what he commanded him? I think not, that is, I think that he does not. Gloss: "What wonder, if a servant does what he owes, whose lot it is to labor and serve continually?" Sirach 33: "Continual labors bow down a servant." And note that the Lord seems to say that the master does not owe thanks to the servant — not that he does not accept his works, since it is said in Sirach thirty-three: "If you have a …

Commentary on Luke, Chapter 17 ↗