Fighting in a relationship is normal, and actually quite healthy. According to Esther Perel, psychotherapist, and author of Mating in Captivity, "fighting is a must." A must because fighting isn't actually the issue. It's the repair that matters.
Where your will power steps in is when you feel the need to say something just-to-piss-him/her-off. Something that hurts, that digs deep and will cause a reaction. Will power is your strongest asset so that you don't say or do something you will regret.