Surely Catalans, and nobody else, must answer that question. Some Catalans consider themselves Spanish and some don't. Many Spaniards consider Catalonia part of Spain, while some don't. But it's clear that a significant number of Catalans feel politically conquered, and resent it. Why should they live under a Spanish government, when their history, culture, and language are not Spanish?
It's a fair question, and one for which western democracies have no easy answer. If democratic voting is sacrosanct, are the results also sacrosanct, whatever the outcome? Do democrats really want democracy?
Ludwig von Mises summed up the problem succinctly in Liberalism1:
The situation of having to belong to a state to which one does not wish to belong is no less onerous if it is the result of an election than if one must endure it as the consequence of a military conquest.