Christ had said that He must be betrayed; Peter replied, "Be it far from Thee, Lord; this shall not happen unto Thee." He was rebuked, but not instructed. On the contrary, when Christ desired to wash his feet, he said, "Thou shall never wash my feet." Again, when he hears, "Thou canst not follow Me now," he saith, "Though all deny Thee, I will not deny Thee." Since then it was likely that he would be lifted up to folly by his practice of contradiction, Jesus next teacheth him not to oppose Him. This too Luke implies, when he telleth us that Christ said, "And I have prayed for thee, that thy faith fail not"; that is, "that thou be not finally lost." In every way teaching him humility, and proving that human nature by itself is nothing.
Source: Homily on the Gospel of John 73