How glorious it feels to hold a grudge. How satisfying and self-indulgent. A true grudge is not about bitterness, hatred, vengeance or anger. Nor is it a feeling that demands action or a solution. Rather, it is the smug pleasure of knowing that you’re entitled to feel wronged—a privately held insight into another person’s flaws. It is also, importantly, an emotional shield—a slight lowering of expectations that immunizes you against being disappointed again.

According to author Sophie Hannah, holding personal grudges can be constructive. In November, she is launching a self-help book dashing the conventional wisdom that grudges are unhealthy, and that functional people always “forgive and forget.” How to Hold a Grudge: From Resentment to Contentment, the Power of Grudges to...