It is a lesson we all need—to let alone the things that do not concern us. He has other ways for others to follow him; all do not go by the same path. It is for each of us to learn the path by which he requires us to follow him, and to follow him in that path. Let us remember our Master's injunction, and we shall be saved from many pitfalls: "What is it to you? You follow me" (John 21:22).
The question Jesus was responding to -- "Lord, what about him?" -- comes from an immature faith. He had just finished commissioning Peter as His vicar, concluding by saying, "Follow me," when Peter turned away and got distracted by the sight of John.
Was Peter simply scatterbrained, perhaps dazed by Jesus' foretelling of his death, or was he just trying to take his mind off the weight of the key he had been given? Maybe even get Jesus to change His mind?
Whatever the case, it's always been easy to take your eyes off Jesus when He is giving you your own personal mission. For that matter, it's always been easy to never look at Jesus long enough to realize He is giving you your own personal mission, and to think our own thinking about others is a mark of virtue, like St. Dominic's tearful question, "Lord, what will become of sinners?"
If with the years I grow less inclined to ask, "Lord, what about him?," it's probably less to do with my own faith maturing than with the lack of success I've had getting that question answered -- particularly when I've tried to answer the question myself. I can't say my faith is all that mature, because I haven't yet come through the night of wrestling with the question, "Lord, what about me?"