The first time I ever heard this passage I thought Jesus was being a little hard on the Apostles. I realized later Jesus was making a point. He was reminding them that doing what is expected of you is nothing to pat yourself on the back about. It is going the extra mile that is important.
This was really driven home for me when I watched an episode of the Chosen. Jesus and his disciples come upon a group of Roman soldiers. Under roman law a soldier could force a Jewish person to carry his things for one mile. The soldiers order Jesus and the disciples to carry their things. When Jesus reaches the one-mile marker the soldier in charge orders him to stop. It is then that Jesus offers to carry his things an extra mile. When Jesus is reminded of the law, he tells all the disciples to say that they offered to carry everything the extra mile. The soldiers then begin to soften and become a little kinder to the disciples. Clearly, the soldiers are bewildered by Jesus’ actions.
Judas, visibly upset by all of this, questions Jesus' decision. Jesus tells Judas to think back to the sermon on the plains. Jesus is reminding him that it is not a sacrifice to do what is expected, the true sacrifice is doing more. Love your enemies, turn the other cheek, do to other what you would have then do onto you. Most of all, do everything out of love for Jesus and not out of a fear of Jesus.
Jesus wants us freely and lovingly doing things for others. He does not expect us to brag about what we do or look for praise. In the end all he wants is for us to go the extra mile because we want to, not because we have to. It is then that we are truly following Jesus.