The Torah (Vayikra 19:17) explicitly states, "Do not hate your brother in your heart." Even the simplest person is commanded not to have envy, hatred, and feelings of revenge and a grudge. This is such an important and basic obligation that the Jewish people's failure in this regard led directly to the destruction of the Second Temple.

Is there anyone you hate without a valid reason? Mentally visualize that hate melting away. In its place, visualize compassion, empathy. Now mentally bless that person. Bless him/her that they should treat you so wonderfully that you are filled with love for that person