One of the major causes of the Enlightenment was the Scientific Revolution. It cannot be overstated how important the work of scientists was to the thinkers of the Enlightenment, because works like Newton's Mathematical Principles demonstrated the existence of eternal, immutable laws of nature (ones that may or may not have anything to do with God) that were completely rational and understandable by humans. Indeed, in many ways the Enlightenment begins with Newton's publication of the Principles in 1687.
Having thus established that the universe was rational, one of the major themes of the Enlightenment was the search for equally immutable and equally rational laws that applied to everything else in nature, most importantly human nature. How do humans learn? How might government be designed to ensure the most felicitous environment for learning and prosperity? If humans are capable of reason, why do they deviate from reasonable behavior so frequently?
Among the other causes of the Enlightenment, perhaps the most important was the significant growth of the urban literate classes, most notably what was called in France the bourgeoisie: the mercantile middle class. Ever since the Renaissance era, elites increasingly acquired at least basic literacy, but by the eighteenth century even artisans and petty merchants in the cities of Central and Western Europe sent their children (especially boys) to schools for at least a few years. There was a real reading public by the eighteenth century that eagerly embraced the new ideas of the Enlightenment and provided a book market for both the official, copyrighted works of Enlightenment philosophy and pirated, illegal ones. That same reading public also eagerly embraced the quintessential new form of fiction of the eighteenth century: the novel, with the reading of novels becoming a major leisure activity of the period.
Thus, the Enlightenment thought took place in the midst of what historians call the “growth of the public sphere.” Newspapers, periodicals, and cheap books became very common during the eighteenth century, which in turn helped the ongoing growth of literacy rates. Simultaneously, there was a full-scale shift away from the sacred languages to the vernaculars (i.e. from Latin to English, Spanish, French, etc.)., which in turn helped to start the spread of the modern state-sponsored vernaculars as spoken languages in regions far from royal capitals. For the first time, large numbers of people acquired at least a basic knowledge of the official language of their state rather than using only their local dialect. Those official languages allowed the transmission of ideas across entire kingdoms. For example, by the time the French Revolution began in the late 1780s, an entire generation of men and women was capable of expressing shared ideas about justice and politics in the official French tongue.
There were various social forums and spaces in which groups of self-styled "enlightened" men and women gathered to discuss the new ideas of the movement. The most significant of these were coffee houses in England and salons in France and Central Europe. Coffee houses, unlike their present-day analogs, charged an entry fee but then provided unlimited coffee to their patrons. Those patrons were from various social classes, and would gather together to discuss the latest ideas and read the periodicals provided by the coffee house (all while becoming increasingly caffeinated). Salons, which were common in the major cities of France and Germany, were more aristocratic gatherings in which major philosophers themselves would often read from their latest works, with the assembled group then engaging in debate and discussion. Salons were noteworthy for being led by women in most cases; aristocratic, educated women were thought to be the best moderators of learned discussion by most Enlightenment thinkers, men and women alike.
Outside of the gatherings at coffee houses and salons, the ideas and themes of the Enlightenment reached much of the reading public through the easy availability of cheap print, and it is also clear that even regular artisans were conversant in many Enlightenment ideas (to cite a single example, one French glassworker, Jacques-Louis Menetra, left a memoir in which he demonstrated his own command of the ideas of the period and even claimed to have chatted over drinks with the great Enlightenment philosopher Jean-Jacques Rousseau). The major thinkers of the Enlightenment considered themselves to be part of a “republic of letters,” similar to the "republic of science" that played such a role in the Scientific Revolution. They wrote voluminous correspondence and often sent one another unpublished manuscripts. Thus, from the thinkers themselves participating in the republic of letters down to artisans trading pirated copies of enlightenment works, the new ideas of the period permeated much of European society.