A nation using its right to self-determination according to the democratic mandate of its people expressed in free, clean elections, as simple as that.
You don't need to ask permission from the other country if that one does not even try to dialogue with you to debate a solution, as India didn't ask, or the Baltics didn't ask, or Ireland didn't ask, or Finland didn't ask, etcetera. Accept that it's difficult next to impossible that a State will grant independance out of their own kindness, specially when that region in particular is one of the four most highly industrialized in Europe. I saw Quebec and Scotland set as examples above; those two countries had someone to dialogue and discuss with, they had an agreed-upon legal referendum, which is something we cannot get since we are time and time again told that we are part of this Spanish nation many people here do not believe or relate to, and also told to engage on a long and complex constitutional process to change the most important document in Spain (the Constitution), which is almost impossible since all Spanish parties would block the proposition and kill it time and time again for decades. Is anyone in this thread seriously thinking that Catalans will stand by this and wait for their whole life for a change that will NOT COME AT ALL.
If democracy was the minority blindly accepting the will of the majority no that many new countries would have emerged as they did in the past two centuries, you don't even need to be an enslaved colony to be entitled to independant, you just need the majority of the population in that region and a clear democratic mandate.
To those who say our culture, heritage and traditions are not endangered perhaps you should speak to a Catalan, or even visit, and perhaps you would leave your bubble and realise Spain is not the ethnically and politically homogeneous state presented abroad. We have (for centuries) experienced an active (although some periods were harder than others) agressive policies towards Catalan language and cultural assimilation. From assigning Spanish nationals to leading administrative positions in Catalan institutions to shifting demographics, the list of grudges is long.
For instance in the past dictatorship (which wasn't even over that long ago, in 1975), Franco prohibited the use of Catalan names. Castilian equivalents had to be used, during his regime, religious services were held in Castilian, and Castilian was the only language permitted to be used in public. Schools were banned from teaching Catalan. Few books were published in Catalan and any that were published were not popularly read because most Catalans could not read Catalan. Images of popular Catalan culture were also banned. Popular symbols of Catalan nationalism, such as statues, portraits, the flag, were all removed from public view. Even the names of streets that were in Catalan were changed to a Castilian name. Spain's effort to suppress our culture was pervasive. Franco prohibited expressions of language, traditional dance and culture, and religious practice, yet he only limited the culture. Although he repressed major Catalan institutions, we were able to resist this repression, and our culture flourished after his death. Faced with a violent despondent regime, we Catalans relied on daily acts of resistance and oral traditions to maintain our culture. After Franco's death the culture and language were encouraged to thrive as the Generalitat, the local democratic government emphasized the importance of maintaining the culture. The increase in the literacy rate and spoken Catalan owe a lot to the effort of the Catalan government to encourage the re-establishment of Catalan culture in social, political, and economic institutions, but also to the daily acts of resistance by the Catalan. I hope you understand the perseverance of a people to maintain their traditions, customs, and language through a history of oppressive monarchies. The Catalan culture survived 36 years of oppression due to everyday resistance and a strong sense of identity that the Catalans have historically harbored in order to maintain their culture in the face of cultural oppression.
And all of this improvements you mentioned in the last decades did not come becuase the masters from Madrid granted them to us, they came as a result of our struggle, resistance and tenacity.