The reason why the dispute concerning the chief place arose amongst the disciples seems to have been, that Peter, James and John, were led apart from the rest into the mountain, and that something secret was there entrusted to them, also that the keys of the kingdom of heaven were promised to Peter, according to Matthew.
Seeing however the thoughts of the disciples, the Lord takes care to heal the desire of glory by humility; for He first, by simply commanding humility, admonishes them that a high station was not to be aimed at.
Wherefore it goes on: “And He sat down, and called the twelve and saith unto them, If any man desire to be first, the same shall be last of all, and servant of all.”
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).