Bonnie Jacobson, a clinical psychologist and adjunct professor at New York University, says that feuding behavior is common among people with fragile personalities. “They feel easily invalidated and react with rage at the smallest infraction. So if a person crosses a boundary into their ‘territory,’ it’s not like a tap on the shoulder; it’s like a punch in the back. If a family member doesn’t side with them, they feel betrayed. Trying to make peace with them often doesn’t work, because it makes them feel justified, and their behavior escalates. You have to be firm and tell them, ‘That’s over the line.’ “