Realization. They might be terrible together and terrible for each other, but once they separate, they're forced to recognize that they're truly broken individuals. They need the Gang to maintain the delusion that they're not that fucked up—even if they're just contributing to another cycle of abuse, it's better than addressing what they've already gone through.
I wrote about extreme character dysfunction at the core of Sunny, and how this theme allows us to explore these characters much more deeply than what's visible on the surface. You can read it on the Paddy's Pub Blog!