PM Pedro Sánchez asked a very tempting question, when he enquired “at what point” did Catalonia and Spain fall out? If what we are looking for is an exact date, history provides us with a wide range of answers of all kinds: 1714, 1939, 2010, 2017... However, I honestly feel that Sánchez is mistaken if he thinks that everything was going fine until one day something happened and the relationship suddenly turned sour.
The problem is not a particular moment in time, but a particular idea of Spain: it has to be plurinational or the relationship will never work. The relationship can only go well if the Spanish state stops treating Catalonia like another province and if it comes to accept its right to decide its political future, too. If you really want a moment in time, I can suggest one: 1977. With the Franco regime very much alive and the military inside the Spanish government, it was possible to include the word "nationalities" in the new Constitution. Now it’s 2020 and the constitutional idea has lain dormant for 43 years, waiting for a champion of dialogue to finally take it further.