John 21:20,21 - Joseph Benson’s Commentary on the Old and New Testaments

Then Peter, turning about Namely, as he followed Jesus; seeth the disciple whom Jesus loved following Silently, and in humble token of his readiness likewise to suffer the greatest extremities in the service of so gracious a Master. Peter, seeing him follow Jesus in the same manner as he himself did, though he was not called to it, saith to Jesus, Lord, what shall this man do What is to become of him? must he, who is now following with me, partake of the like sufferings, and in like manner testify his love by dying for thee? “There is a peculiar spirit and tenderness in this plain passage. Christ orders Peter to follow him, in token of his readiness to be crucified in his cause. John stays not for the call, he rises and follows him too; but says not one word of his own love or zeal. He chose that the action only should speak this; and even when he records the circumstance, he tells us not what the action meant, but with great simplicity relates the fact only. If here and there a generous heart sees and emulates it, be it so; but he is not solicitous that men should admire it. It was addressed to his beloved Master, and it was enough that he understood it.” Doddridge.