Catalans speak their own language. They are richer than their fellow citizens in the rest of Spain, and thus contribute more to the national budget than they receive back. They carry unhappy memories of domination by centralizing governments in Madrid. The Franco dictatorship of 1939-1975 harshly suppressed their culture, identity, and political and civil liberties. It’s a pure accident of history that Catalonia ended up inside the Kingdom of Spain at all. Portugal successfully broke away in the 1640s. Had the Thirty Years War, the War of Spanish Succession, or the Napoleonic wars taken a slightly different turn, Catalonia might well have followed.