Peter had a big horizon.  He could think ‘out of the box’.  And that would prove to be a blessing at Pentecost when the Holy Spirit used his big personality to be the first preacher of the gospel.  However, on this occasion, having been told by the Lord that his life would end by crucifixion (John 21:18-19), Peter then wanted to know if John would also be crucified. But Jesus refused to tell him John’s future, and only reiterated that Peter must follow Jesus very closely from now on.
 
Jesus said that John would do whatever He chose for him; but that information was private.  Even if John was to remain alive until the Second Coming, what difference should that make to Peter?  It sounded to the disciples like a promise of John’s longevity (although it was not).  Peter was ordered not to be deflected from following Jesus - however difficult it might be, and whatever might happen to the other disciples.  John’s future was not his business … “What is that to you?” Jesus said.
 
Although it is important to be aware of people around us and to be concerned for them, we can sometimes get more involved in the lives of others than getting on with the work which the Lord has set before us. Whatever our interest in others, and our sense of responsibility for them, our first priority is to follow the Lord ourselves.

Even our Bible readings should find their first target inside our own hearts, rather than thinking, ‘that is a good verse for someone else’!  Each of us is solely accountable to the Lord for our own lives and for fulfilling the commissions He gives to us.  Most of the time, God’s dealings with other people are not our business; but it is always right for us to follow Jesus wholeheartedly, whatever the cost.