"People should be allowed self-determination and popular sovereignty when I agree with them!"
Basically this, honestly. Whether independence is 'justified' or 'allowed' is entirely a question of whether you think their reasons are valid. We declared independence from Britain because they were stuffbags levying predatory taxes against us, which was a pretty good reason for independence. But we simultaneously fought a war against the Confederacy because they were a bunch of stuffbags declaring independence just so they could continue to keep black people as chattel. There is no universally-accepted code of conduct for what justifiably constitutes a reason for independence. It's whatever the hell you agree with.
I'm inclined to side against Catalan independence since it seems like the gist is that they're a bunch of rich starfishs who aren't happy with the fact that the rest of the country isn't as rich as they are. So instead of, say, trying to do something like improving infrastructure, they just want to jump ship.
