One way to look at it is that the sovereign country of Spain has a right to split itself up in two countries. Therefore all Spaniards should have a say.

Another way to look at it is that every sub-entity of any country has a right to unilaterally declare itself independent for whatever reason (often because it is wealthier).

I think neither of these views is sufficient. We have to accept that this is a tricky question that is very difficult to solve in a principled way.