BBS is a tragedy in many ways, but one of the things that gets me is that the catalyst, the one big thing that caused this mess to begin with, is so avoidable.

Like, the story starts primarily because Terra starts to fear himself (because of the darkness he has unwittingly 'used') and this causes a spiral of doubt and obsession. A spiral which he could've escaped from or maybe not even started if he just 1) had been taught the balance between light and dark properly (that darkness is more than just unquestionable evil, that light is more than unquestionable good, that they can and should co-exist in balance), and 2) had consistent socialization from more than the same 3 people.

Like. BBS is a cautionary tale about what happens when people aren't exposed to a wide enough variety of people and, therefore, end up naive and overly trusting because they never learned that people have ulterior motives. Terra is consistently manipulated and used by people literally everywhere he goes, because he can't see that they might not have the best intentions! He can't see the red flags because he doesn't know there are red flags to look for!

Xehanort knows this. It's why he targeted Terra to begin with: because he would be susceptable to his words. The main reason he couldn't use Aqua is because she was too confident in her own values, she didn't have the seed of doubt that Terra had. It's the only thing that prevents her from being used the same way he is, even though she is just as naive and trusting as he is.

Ven probably has it the worst socialization wise. Unlike Terra and Aqua who, presumably, has/had a family and life outside of the Land of Departure prior to beginning training there, Ven doesn't remember anything before then (not that he had much he'd be happy to remember anyway). And since he's the youngest and most 'fragile' one (due to being in recovery for most of his time here), he gets somewhat coddled and shielded by everyone else. Not to mention the way Eraqus completely forbid him from leaving and never intented for him to ever see worlds outside the Land of Departure. Sure, Terra and Aqua aren't typically supposed to leave either, but at least they'd be allowed in certain situations. Ven wouldn't.

Ven isn't allowed to interact with anyone outside of home. He's not allowed anything that involves the outer worlds. (He and Naminé are a bit alike in that sense. Though at least the people he's stuck with are nice to Ven and do genuinely care for him, unlike Naminé...)

BBS happens largely because Eraqus failed as both a teacher and a parental figure to all of them, but Terra most of all.