John 21:21

‘Peter therefore seeing him says to Jesus, “Lord and what shall this man do?” '

Peter then asked Jesus about John's future. What caused Peter to do this? The impression given is that he said it on the spur of the moment when his glance happened to fall on John. It may have been his way of ‘softening the blow' of the preceding words by turning his mind to something else, (the thought could not have been pleasant). Or it may have been because he was impetuously enthused to know what sort of violent death other faithful followers would suffer, that they also may glorify God. But he was firmly told that that had nothing to do with him. What Jesus has told him was for his restoration in his own eyes and the eyes of his fellow-disciples, not just for the sake of knowing the future. That was best to be left in God's hands.