Catalonia: Why are they trying to declare independence?

Recently, Catalonia (Catalunya in Catalan), an autonomous community in the Kingdom of Spain, has been in the news after a controversial referendum to declare independence from Madrid. Both sides seek to discredit the other: the Spanish loyalists point out that only pro-independence Catalans turned out for the referendum (declared unconstitutional by Madrid) and that less than half of registered voters voted. The Catalan separatists also rightly highlight the disturbing levels of police violence against voters as roughly 900 people were injured and another 750,000 votes were uncounted because of closed polling stations and confiscations.