Basically, Catalan thinks of itself as an autonomous part of Spain, with their own unique cultural heritage. All (or most?) schools even teach in the Catalan language. They also (from how the USA news and the BBC seem to report on it) think of themselves as better? (for lack of a better word) than the rest of the country - some areas can have a higher standard of living, higher wages, and are more wealthy than
some/many? parts of the rest of Spain (and the news reports like to play clips of Catalans saying things like how they're "tired of paying for the rest of the country").
There is also a backdrop going back to the Franco dictatorship (ended mid-70's?) where they were basically put on notice by the government (i.e. crackdowns), had thousands of people killed, thousands more put into prison, mass exodus for many families as well – and other bad shit like that. So since that's still semi-recent memory for some of their 50+ population, I'm sure that doesn't make them feel good about being part of Spain either.
So lately they had an unofficial vote ("unofficial" according to Spain, "official" according to Catalan) on basically if they were going to succeed from Spain (conflicting reports on what percentage actually voted, and/or what the results actually were), and Spain police came in and beat the shit out of a lot of people trying to vote, riots, protestors, etc. Now the "president" of Catalan is backpedaling it seems, since he doesn't want to go to jail...so we'll see what happens. (and any native Spanish, Catalan people - please correct me if I have any of this wrong - I'm simply an imperialistic American pig-dog who pays attention to the news