Knowing exactly what to say to hurt someone and not saying it is real power.

Anyone close to you hands you a map of where they are vulnerable just by being close. Holding that knowledge and refusing to use it, especially in the middle of an argument, is one of the clearest forms of self-control there is. Using it wins the exchange and permanently changes what they will tell you again. Restraint here is not weakness; it is what makes closeness possible at all.

When an argument heats up and the perfect cruel line arrives, notice it and let it pass. Say the plainer, less effective thing instead. Nobody regrets the sentence they did not use.