[Image Description: the Estelada, Independent Catalan flag; by tooooo - Own work, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=739696 ]
I am flying the Estelada not because I think that Catalonia/Catalunya/Cataluña should go independent from Spain, but because I believe in the inherent right to self-determination of all peoples, which has also been a concern of international law since 1860. In modern times, as I understand the matter, all new states have only been considered such after the United Nations has agreed to same.
In all this, one should recall that modern nation-states as we understand them are all relatively new, only since the nineteenth century.
My personal opinion is that the Catalans should be better within the Spanish association, but only if Madrid shares out resources more fairly and treats Catalans with more respect.
However, under Rajoy's PP minority government, I cannot see that happening anytime soon.
Spain is too much controlled by Madrid - the seat of the Spanish centralised government. The State needs to be looking at a much looser association, if it does not want the Catalans, Galicians, Andalusians and maybe even Valencianos seceding. I should suggest devolving as many powers as possible to the regions, etc. and only controlling from the centre defence and the highest level diplomacy, such as for treaties, etc. That awful EU neologism 'subsidiarity' should be the mark of the modern, democratic State.
And that also goes for the UK too, where Scotland is treated not dissimilarly by the British equivalent of Spain's Partido Popular, our Conservatives - commonly referred to as the Tories - who are likewise a minority government.