1 When the Lord was washing the disciples' feet, “He comes to Simon Peter; and Peter says unto Him, Lord, dost Thou wash my feet?” For who would not be filled with fear at having his feet washed by the Son of God? Although, therefore, it was a piece of the greatest audacity for the servant to contradict his Lord, the creature his God; yet Peter preferred doing this to the suffering of his feet to be washed by his Lord and God. Nor ought we to think that Peter was one among others who so expressed their fear and refusal, seeing that others before him had suffered it to be done to themselves with cheerfulness and equanimity.
For it is easier so to understand the words of the Gospel, because that, after saying, “He began to wash the disciples' feet, and to wipe them with the towel wherewith He was girded,” it is then added, “Then comes He to Simon Peter,” as if He had already washed the feet of some, and after them had now come to the first of them all. For who can fail to know that the most blessed Peter was the first of the apostles? But we are not so to understand it, that it was after some others that He came to him; but that He began with him. When, therefore, He began to wash the disciples' feet, He came to him with whom He began, namely, to Peter; and then Peter took fright at what any one of them might have been frightened, and said, “Lord, dost Thou wash my feet?”
What is implied in this “Thou”? And what in “my”? These are subjects for thought rather than for speech; lest perchance any adequate conception the soul may have formed of such words may fail of explanation in the utterance.
Source: Tractates on the Gospel of John (New Advent)