John 21:21 - Ellicott's Commentary On The Whole Bible

Lord, and what shall this man do? — The motive prompting this question was probably that of loving interest in the future of his friend. It may well be that the two friends, in the sadness of the dark days through which they had passed, had talked together of what their Master’s predictions of the future meant, and had wondered what there was in store for themselves. They knew the world was to hate them as it had hated Him, and they never knew what its hatred for Him was. One of them had learnt that he was to follow his Lord in death as in life, and he now sees the other following them as they draw apart from the group, and would fain know the future of his friend as he knew his own.