If you hate a person, you hate something in him that is part of yourself. What isn't part of ourselves doesn't disturb us.Herman Hesse
I'm not sure how accurate this statement is and doubt it could be well verified, but it gives me a healthy pause and cause to evaluate my own sentiments towards others. And any check on the tongue is usually good.
It also comes amusingly to mind when friends and family share their partisan bile with me. I try to keep from responding instead waiting to change the subject, and think of this quote which provides a little amusement to protect against the bitterness so many loved ones with to share.