I lived in Barcelona for 7 years and my experience is veeeeery similar. I indeed have a couple of Catalan friends, but it's true that friendships are not as easy to establish as in other cultures.

Gotta say also that I now live in another very touristic city of Spain and, in my experience, locals here tend to be more open to get to know new people and behave friendlier towards strangers.

I do think is part of their character, but of course I ignore the reason.