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.

This quote suggests that if you have strong negative feelings towards someone, it is likely because that person has qualities that you recognize and dislike in yourself. This can make the person more irritating or annoying to you than someone who does not possess those qualities. You are more likely to see and be disturbed by the same qualities in someone else that you do not like in yourself.