One Doesn't Have to Forgive
Forgiveness is widely treated as a duty, but the duty is inherited. The idea that one must forgive comes mainly from Christianity, where forgiving others is commanded and tied to one's own forgiveness by a God. Remove that theological frame and the obligation loses its foundation. In a secular ethics, responses to being wronged have to be justified by their effects on people, not by divine instruction, and once the question is framed that way, forgiveness becomes one option among several rather than a requirement. A person who declines to forgive has not failed at anything; they have simply chosen a different, equally available response.
The common objection is psychological: unforgiveness supposedly means carrying anger that corrodes you. But this conflates two different things. Letting go of resentment, rumination, and preoccupation with a wrong is good for you, and the research supports that. Granting the wrongdoer absolution is a separate act. You can stop thinking about someone, stop being angry, and get on with your life without ever forgiving them. The psychological benefits attributed to forgiveness largely come from the letting go, which does not require the pardon. Indifference, acceptance, and distance do the same protective work.
There are also positive reasons to withhold forgiveness. It is OK to judge people and their actions. Judgment is how we track character, and some acts reveal character. Declining to forgive can be an accurate moral response, a way of continuing to take the wrong seriously when the person who committed it has not acknowledged it, made amends, or changed. Automatic forgiveness removes one of the few costs of treating people badly. And the social pressure to forgive falls hardest on the person who was harmed; it quietly relocates the moral burden from the wrongdoer, who did something, to the victim, who now owes something. That relocation should be resisted, not praised.
Finally, whatever value forgiveness may have depends on its being freely given. When someone forgives because they genuinely no longer hold the wrong against the person, that can be meaningful to both parties. When someone forgives because they were told they must, it is a performance extracted under pressure, and it means nothing. Forgiveness can matter if withholding it is a real option. So even people who value forgiveness may agree that no one has to forgive; the alternative makes the act empty.
References
Griswold, C. L. (2007). Forgiveness: A philosophical exploration. Cambridge University Press.
Murphy, J. G. (2003). Getting even: Forgiveness and its limits. Oxford University Press.
Nolen-Hoeksema, S., Wisco, B. E., & Lyubomirsky, S. (2008). Rethinking rumination. Perspectives on Psychological Science, 3(5), 400–424. https://doi.org/10.1111/j.1745-6924.2008.00088.x
Nussbaum, M. C. (2016). Anger and forgiveness: Resentment, generosity, justice. Oxford University Press.
Witvliet, C. V. O., Ludwig, T. E., & Vander Laan, K. L. (2001). Granting forgiveness or harboring grudges: Implications for emotion, physiology, and health. Psychological Science, 12(2), 117–123. https://doi.org/10.1111/1467-9280.00320