"When Peter saw (the disciple), he said to Jesus, 'Lord, what about him?' Jesus said to him, 'If it is my will that he remain until I come, what is that to you? Follow me!'"
It must have been bittersweet. Peter, who had denied & abandoned Jesus when he needed it most, had just been forgiven & restored by the Master. The Risen Christ had come to Peter and the disciples while they were fishing. He'd given them a few fishing tips (extremely effective, as always!), BBQ'd breakfast for them on the beach, and had a heart-to-heart with Peter. Powerful! But Jesus also told Peter he would (eventually) die in his service of God. So it's only natural that Peter might point to his buddy sitting next to him and ask Jesus about his future (v.21): "Lord, what about him?"
We love comparisons, don't we? We have top 10 lists "out the wazoo." We rank anything & everything, from caffeinated beverages to sports teams to music. We analyze... size up... examine things side to side, etc. Even (especially!?) people. How do we compare with him/her/them?
But Jesus cuts this off quickly. He tells Peter not to worry about it. Peter's job is simple: "Follow me!" Jesus says. Period. Don't waste time & energy comparing myself to anyone else. Follow Jesus. Focus on how he's called me to live out my faith. My faith. No one else's. Sounds kinda simple, doesn't it? (Yah, I know... I would have asked the same question Peter did, too.)