Peter turned and saw the disciple following whom Jesus loved, the one who had also reclined upon his chest during the supper and had said, "Master, who is the one who will betray you?" When Peter saw him, he said to Jesus, "Lord, what about him?" Jesus said to him, "What if I want him to remain until I come? What concern is it of yours? You follow me." So the word spread among the brothers that that disciple would not die. But Jesus had not told him that he would not die, just "What if I want him to remain until I come? (What concern is it of yours?)" It is this disciple who testifies to these things and has written them, and we know that his testimony is true. There are also many other things that Jesus did, but if these were to be described individually, I do not think the whole world would contain the books that would be written. (Jn 21:20-25)
Peter and John had many things to do in common and they have done so from the beginning. Even in the last chapters (chapters 13, 18, 20 and 21) they have been important and they have been on the scene many times. Now their paths are taking different turns. But they should not look over their shoulders to see what the other is doing. They should not wonder why the other is different. Vocation is personal though it is from the same Lord and though they have been together with the Lord (in their formation time)
Peter’s question about John is natural. They have done so much, side by side. Peter expected that John also would face the same end. Most important thing for Peter is to learn that God has different plan for each of them. Christian obedience is personal and the mission given to the disciples is personal. We are called to follow Jesus wherever he leads us, not wherever he leads the person next to us.
It is not Peter’s business to know about John. Peter should follow Jesus on his own and his following should not be conditioned by the followings of the other. The words of Jesus to Peter are quite strong: “What concern is it of yours?” Peter was called to follow Jesus and only Jesus.