"But Lord, what about this man?" (John 21:21). It is difficult to know what exactly Peter's motive was in asking such a question. Was he merely curious about what the end of his friend would be? Did he wonder if others (especially one like John who was very close to the Lord) would share a similar fate as himself?
"If I will that he remain till I come, what is that to you? You follow Me" (John 21:22). There appears to be a mild rebuke in Jesus' response. He is reminding Peter that each person's position and responsibility before the Lord is individually based and does not directly involve anyone else. Peter was to follow the Lord faithfully (as was John), regardless of the blessings given or responsibilities entrusted to another. It was none of Peter's business whether John's earthly lot was easier or more difficult than his own. His task was to be faithful down the pathway that the Lord would lead him.
"Then this saying went out among the brethren that this disciple would not die. Yet Jesus did not say to him that he would not die, but, 'If I will that he remain till I come, what is that to you?'" (John 21:23). John is quick to point out what Jesus did not say. The Lord never promised that John would live forever physically, but a rumor was circulated to that effect. This narrative certainly reveals how readily people will leap to wrong conclusions. John must have felt obligated to dispel the rumor since it would negatively affect the faith of any that believed it (if indeed he did die before the Lord returned, which we know he most certainly did).
"This is the disciple who testifies of these things, and wrote these things; and we know that his testimony is true" (John 21:24). That which is contained in this gospel account is eyewitness testimony of the life, acts, and teachings of Jesus. It was not information that was passed on orally from a friend who heard a friend who heard a friend, etc.
Some doubt that John wrote these verses since "we" is used (instead of "I" or "he," cf. 19:35). However, in the first epistle of John, the first chapter opens with and continues to use the editorial plural, but the second chapter drops in the first person singular. Thus, the use of "we" here is no certain proof that these verses were not written by John himself.
"And there are also many other things that Jesus did, which if they were written one by one, I suppose that even the world itself could not contain the books that would be written. Amen" (John 21:25). John, of course, is exaggerating here. All of the things that Jesus did while on Earth could have easily been recorded in a finite amount of space. John's point is that what actually was recorded is such a very small amount of all that Jesus did (cf. 20:30,31).
John has presented his case, and now it is up to the reader to either choose to follow Jesus as the Son of God, or reject Him. Was Jesus the Son of God who went about doing signs and wonders that proved His claim, or was He a Galilean peasant crucified for heresy at the instigation of the Jewish religious leaders? This is a decision that everyone must make, and they will live with the consequences of their decision for eternity.