That image, found on FB, can be very lightening about the reason of so much independence desire in Catalonia... Later I'll try to explain better the roots (only a light summary), but it's said that an image can be more powerfull a thousand words... so here I go!! (below the image, the explanation!!)
First frame: a guy with the normal Catalan flag. "Oigan" = "hello", "hear to me"...
Second frame: all things usually said to Catalan by certain Spanish media (and people who hear that media), like "unsupportive", "cretin", "buffon", "blackmailer", "Polish"*...
Third frame: more insults: "nazis", "separatists", "selfish", "pest", "parasites"... The guy leaves the Catalan flag and...
Last frame: the same guy, who didn't feel independentist, with the "Estelada" (the independentist Catalan flag)
That's one of the reasons I was at that demonstration
*Polish: yes, it's said like an insult. Don't ask why, Catalan people are called "Polacos" (Polish) in a pejorative way. A theory is that, as we speak another language, and they don't understand this language (even when Spanish and Catalan are very similar!), it's like if we were talking Polish. We're sometimes said "talk (to me) in Christian!" (as if we were talking some barbarian language non understanable for Christians???).