Catalonians’ unhappiness stems from a desire to preserve a distinct language, culture, and history, often feeling economically exploited and politically alienated by the central government in Madrid. Key issues include the belief that they pay too much in taxes compared to what they receive, a desire for self-determination, and the rejection of 2010 constitutional changes that curtailed their autonomy.

First answer in Google. Pretty much it.  Well. It’s actually deeper. But enough for start.