Earlier quoted context omitted.
To the EU this is an internal matter, plus, most of the countries of the EU have regions with similar intentions (of secession), it would be like shooting themselves in the foot. Plus new state members of the EU have to be approved by every current member IIRC.
The EU is absolutely happy to interfere with internal matters when it suits them, to a huge degree. Look at how they have been treating Poland and its reforms of its court system. Or look at how the EU is trade sanctioning Switzerland due to a dispute over Swiss internal working regulations. The Commission is all about "European values" and how they're going to impose penalties when a country is doing something unali…
Someone wants to have a European Civil Code, and they say "but I already know the rules to trade with 30 million people". When you cut up the countries, they say "but if I want to trade with someone an hour's drive away, the rules are different - it's better to unify".
There's more boundaries where rules change, so there's more motivation to smoothe them out.
There's less power for each individual government, because their voice is 1 in 60 instead of 1 in 28. You'll quickly come to an understanding that the rules of European decision making have to be standard federal rules, rather than some compromise between federal and international rules.
The motivation for European integrationists is absolutely and solely for Catalonia and other places to become direct members of Europe.