18 Truly, truly, I say to you, when you were young, you used to dress yourself and walk wherever you wanted, but when you are old, you will stretch out your hands, and another will dress you and carry you where you do not want to go.” 19 (This he said to show by what kind of death he was to glorify God.) And after saying this he said to him,“Follow me.”
20 Peter turned and saw the disciple whom Jesus loved following them, the one who also had leaned back against him during the supper and had said, “Lord, who is it that is going to betray you?”21 When Peter saw him, he said to Jesus, “Lord, what about this man?”22 Jesus said to him, “If it is my will that he remain until I come,what is that to you? You follow me!”
Peter was understandably disappointed to find out that he would be killed for his service to God in the future as Jesus points out in vs. 18.
Peter immediately, however, turns his concern to John asking, "What about John? Will he have to die too? If I have to die, I think he should have to too. It’s not fair if I have to die and he doesn’t." (or something like that ~ VVCSV)
Jesus’ response is to say, “What’s that to you?”
Because of Jesus' response, some people interpretted that to mean that John would never die. But John is quick to point out that this was not the point of what Jesus said to Peter.
Constantly comparing ourselves to others is always an occassion for pride or despair. We either get cocky because we judge our neighbors as being beneath or behind us OR we get depressed because we judge our neighbors as being above and beyond our ability to reach.
God rewards and distributes according to His wisdom, mercy and purpose. If He gives some things to some people and some other things to some other people, what's that to you?
1 Corinthians 7:17
However, each one must live his life in the situation the Lord assigned when God called him. This is what I command in all the churches.
(I know the context of this 1 Cor 7 passage is not exactly an apples to apples situation, but I believe it does speak to the general principle of our duty to respond to what God has designed/assigned for us specifically.)