Jesus answered, “If I want him to remain alive until I return, what is that to you? You must follow me.”
Jesus strongly told Peter that it is none of his business. He followed with a repetition of the command in verse (21:19) except there is a slight change in the wording and now there is emphasis on the word “You.” The difference between the two commands is as the difference in “Follow Me!” and “You follow Me!” These are things outside of what Peter should be concerned about and he must quit being distracted by them and concentrate on doing what he should be doing (Rom 14:4). Our directive from Jesus is to follow Him and not worry about what His plans are for others.
John 21:19 Jesus said this to indicate the kind of death by which Peter would glorify God. Then he said to him, “Follow me!”
Romans 14:4 Who are you to judge someone else’s servant? To his own master he stands or falls. And he will stand, for the Lord is able to make him stand.