Saying because they're brats is not completely wrong. Although that's the magic of language, just because the information is right, the emotion conveyed aren't always as intended, and in this case, judging negatively.
They don't have experience and anyone else to turns to. When a problem finally came, they can't talk about it. Really, it's not as easy as comment section usually make, like characters can just "open a communication" without any build up and hope it's the magic bullet to solve any issues. Then after becoming estranged, breaking up is just a closure for them, so they can go on with their life (until parents' marriage happens).