People
Family quarrels are bitter things. They don't go according to any rules. They're not like aches or wounds; they're more like splits in the skin that won't heal because there's not enough material.
— F. Scott Fitzgerald

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
Fitzgerald says family fights are nasty and rule-free, and they aren't like ordinary injuries. They're like skin that's split and can't close because there isn't enough material to cover the gap, a vivid image of a hurt that lacks what it needs to heal.
Context reviewed