Not defining the contents of the commandment, but expressing intent. Compare Joh 13:34, and see on Joh 15:13.
His brother
“To the persecutor Saul, Christ said, 'Saul, Saul, why persecutest thou Me? I have ascended into heaven, yet still I lie upon earth. Here I sit at the right hand of the Father; there I still hunger, thirst, and am a stranger'” (Augustine).