But ye shall not be so - Christ here takes occasion to explain the nature of his kingdom. He assures them that it is established on different principles from those of the world; that his subjects were not to expect titles, and power, and offices of pomp in his kingdom. He that would be most advanced in "his" kingdom would be he that was most humble; and in order to show them this, he took a towel and girded himself after the manner of a servant, and washed their feet, to show them what ought to be their feelings toward each other. See John 13:4-17.
He that sitteth at meat - The master of the feast, or one of his guests.
But I am among you - This was said in connection with his washing their feet. He "showed" them how they ought to feel and act toward each other. "They" ought, therefore, not to aim at office and power, but to be humble, and serve and aid one another.

Let him be as the younger - Dr. Lightfoot justly conjectures that Peter was the eldest of all the disciples; and he supposes that the strife was kindled between him and the sons of Zebedee, James and John. These three disciples were those whom Christ had distinguished by peculiar marks of his favor; and therefore it is natural to conclude that the strife lay between these three, the two brothers and Peter. Shall we or Peter be at the head? Neither, says our Lord. Let him, Peter, who is chief (ὁ μειζων, the eldest) among you, be as, John, ὁ νεωτερος, the younger. The younger part of the disciples do not appear to have taken any part in this contention; and our Lord shows Peter, and the sons of Zebedee, that they must be as unambitious as the younger in order to be acknowledged as his disciples. Dr. Lightfoot thinks that Peter was the mover of this strife, and therefore our Lord rebukes him by name.

But ye shall not be so,.... See Gill on Matthew 20:26.
but he that is greatest among you; in age or gifts, or would be thought to be the greatest, who is most ambitious of grandeur and authority, which perhaps might be Peter's case, who was the oldest man:
let him be as the younger; as John, the beloved disciple, who was the youngest of them; and be as modest, and as humble as he, and reckon himself as in his place, and condescend to men of low estates, and esteem each other, even the youngest, better than himself. So the phrase, (r), "both greater and lesser", is used of the elder and younger.
And he that is chief; that is, a spiritual ruler and governor in the church of God, as all the disciples were:
as he that doth serve; for the apostles and ministers of the word, though they are over others in the Lord, and have the rule over them, yet they are servants for Jesus' sake, and so ought to reckon themselves; See Gill on Matthew 20:27.
(r) Targum in 2 Chron. xxxi. 15.

But ye . . . not--Of how little avail has this condemnation of "lordship" and vain titles been against the vanity of Christian ecclesiastics?

But ye are to be benefactors to mankind, not by governing, but by serving.