8 Thus “there have been multiplied above the hairs of My head they that hate Me gratis”. How multiplied? So as that they might add to themselves even one out of the twelve. “There have been multiplied above the hairs of My head they that hate Me for nought.” With the hairs of His head He has compared His enemies. With reason they were shorn when in the place of Calvary He was crucified. Let the members accept this voice, let them learn to be hated gratis. For now, O Christian, if it must needs be that the world hate you, why do you not make it hate you gratis, in order that in the Body of your Lord and in this Psalm sent before concerning Him, you may acknowledge your own voice? How shall it come to pass that the world hate you gratis? If you no wise hurtest any one, and art still hated: for this is gratis, without cause...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)