Being a forgiving person is part of the path of practice towards Enlightenment. Forgiveness liberates both pain and guilt. It unties knots of tensed negative karma, stops ill seeds from growing into greater hatred. Being unforgiving is nurturing our sense of hatred - which is no less than one of the three poisons that keep us swirling in Samsara's vicious cycles.
However, forgiving another does not mean we allow the same wrong to be repeated. We have to realise that while it could be our negative karma that we are wronged, we should put in the effort to prevent the person who did us wrong from doing more harm, creating more negative karma for himself. By being complacent, we could well be creating more negative karma for ourselves! Remember - forgiveness is "for giving". It is a gift of reassurance - it sets both parties' hearts and minds at peace.