Historically Catalonia is not Spain but a territory that spans the part of Spain near the Pyrenees as well as Territory in the relative place in France. They have their own culture and history in many ways although there is also plenty of cross pollination with the Spanish. However you probably know this.

If then you are asking why the current push for independence it mostly comes from the aftermath of Francoism. In order to forge national unity and robust Spanish identity Franco was adversarial toward the languages and cultures of what are now the autonomous regions. Catalan was no longer to be spoken or taught in any official capacity and was largely treated with contempt.

After the fall of the Francoist regime the Catalan people began reclaiming their language and culture. This generation is the first real generation of recipients of a proud Catalan education. Coupled with the relative wealth of Catalonia compared to the rest of Spain and the poor economy independence has become an attractive option.