LEV548 [This verse] says one may not hate in one's heart. The hatred, when permitted, must be in the open and the "enemy" must be openly confronted, according to Maimonides (Sefer Hamitzvot #302). Nachmanides (commentary on this verse) says that if a person confronts the object of his or her hatred, in all probability the reason for the anger will be out in the open, which will lead to a reconciliation. Even if a reconciliation does not come about, the person hating will feel better.