My expertise only goes to the 15th century, when Catalonia was already part of the Kingdom of Aragon.

Aragon was a kingdom composed of a series of territories along the Eastern part of the Iberian peninsula. By the mid-15th century (1400s), it was a relatively prosperous late Medieval kingdom oriented largely towards trade in the Mediterranean.

Viceroys ruled over Southern Italian possessions and merchant vessels competed with Italian merchant city-states.

But all was not well. Waves of bubonic plague devastated the European world in general, but in particular the tightly interconnected commercial networks of the Mediterranean. As Janet Abu-Lughod notes in her excellent book, Before European Hegemony, the plague devastated the economies of the region. Ships lacked sailors, the fields lacked field hands, urban artisans left their looms and shops unattended as their corpses piled up.

Both Genoa and Venice were hard pressed to maintain their commercial and military strength during this period. A lesser known victim of this same mess was Aragon.

They found themselves overextended, weak, and worried about the long-term fate of their kingdom.

Enter Castile. The Kingdom of Castile was Aragon's neighbor to the West. It contained a series of territories from the Northern coast of modern Spain all the way to South-Central Spain. It had been expanding successfully over centuries in a push southward against the ever shrinking Muslim controlled territories of Southern Iberia.

It was a warrior society, with caballeros and hidalgos taking part in wars both for pious reasons and for the much more tangible reason of receiving lands, titles, and wealth in compensation for their services in expanding the kingdom.

While Aragon contained some of Spain's most fertile lands and industrious cities, Castile was still largely a rural society which depended on herding sheep to compensate for the harsh and less fertile planes in the center of Iberia.

Thanks to Castile's expansion and the massive loss of life in Aragon due to the plague, the former was far better off in terms of raw numbers and in terms of military strength.

Factions in the respective courts of Aragon and Castile successfully conspired to have the two monarchs, Ferdinand of Aragon and Isabel of Castile, wed. However, this was not an equal union. The kingdom's were not truly united, Ferdinand never obtained a legal claim on Castile directly, and each continued to have its respective laws, weights, customs, and languages (what we today call Spanish is actually castellano, castillian, while in Catalonia they speak catalán).

It should here be noted that the Kingdom of Aragon had far more restrictions on the rights and powers of the monarchs. Their rule was more contractual and limited. Legally, their continued rule was dependent on the respect for the customs and rights of their subjects. This was not an Absolutist Monarchy. While Castile's real control was limited in many ways, the monarch was not as legally handicapped in theory from taking radical measures vis a vis its subjects, although in practice the monarch might prefer inaction due to the strength of its aristocrats who were jealous defenders of their rights and privileges.

In history books, the result of this union is often simply just called "Spain", even though in practice the name spread only later.

After a brief interlude as a part of the Holy Roman Empire under Charles I/V, son of Ferdinand and Isabel, Spain was given to one of Charles' sons, Philip II, who then proceeded to rule both Castile and Aragon (still legally separate entities) along with a number of other possessions, including the Netherlands.

British historian Perry Anderson writes in The Lineages of the Absolutist State that the conquest of the Americas and the massive influx of gold and silver from the colonies there slowed down the process of political and administrative centralization which Castilian and Aragonese monarchs had been trying achieve. Centralization was a long, difficult, costly process which could inspire rebellions. Compensating for local inefficiency/weakness with constant shipments of precious metals from across the sea was far easier and preferable, in the short term.

The next several centuries were marked by a very slow and largely incomplete process of integration.

I'm unfortunately far better read in the 16th century and early 19th centuries than I am in the 18th, so the exact way that the Bourbon Reforms impacted Aragon's incorporation into Spain is beyond me. Sorry.

What I can tell you for certain is that the failure to completely integrate its diverse possessions left the Spanish monarchy quite crippled. It could demand taxes and levies of soldiers from Castile or elsewhere far more easily than Aragon, for example. As John Elliot has pointed out in Imperial Spain: 1469-1716, this led to increasing domestic unrest as Castilian peasants felt crushed under the weight of ever increasing taxes and constant levies for bloody wars across Europe. At the same time this is happening, we're also seeing the long term impact of gold and silver shipments from the Americas, which led to inflation and a major economic crisis in the late 16th and early 17th centuries.

There are a few key gaps in this explanation, especially the impact of the disastrous war between France and Spain the mid-17th century where part of Catalonia was ceded to France, but I don't feel comfortable giving the Cliff Notes version of that without knowing the substance behind it. Hope this at least partially answers your question.