"What about him?" he asked. I wonder what his tone was. Was he just curious about John's assignment? Did he fear that John would be left out? Did he think John was being favored?
Jesus dismissed the question. "What is that to you?" John had a different assignment than Peter. Jesus did not treat everyone the same. He loved everyone, but the invitation to come and follow meant different things for different people.
Then he shares with Peter that his future holds some distressing events, and he continues with indications of a different path for John. He doesn't indicate that either one will be more or less favored; they will just be different.
Jesus says "What is it to you if he stays till I come. You follow me."
We have he same sort of questions Peter had. What about my sister or husband or child? I have friends who need to be here; what about them? Jesus looks at us with the same questions he asked of Peter: What is that to you? It won't change your mission to check out the mission of others. You must be obedient to your own call. You don't get see if they have a better deal than you. Jesus calls you to serve in the place chosen for you.
We each have a place of service in obedience to the call of Jesus. The greatest joy and ultimate fulfillment of our lives comes in obeying the call. It really doesn't matter what others have or do or what their call is. Like Peter, we are called to follow him.