I was just reading about Ottoman history for fun, and I noticed the main cause of their fall was nationalism. I noticed this through all of Europe during the 19th and 20th century, Why did everybody, such as the Serbs, Hungarians, and Greeks want their independence from their occupants, but not before this period? What inspired this rise of nationalism?
You could possibly cite the rise of enlightenment thinking as part of it. Many of these territories were under control of foreign empires including the Prussians, the Hapsburgs, the Russians, or the Holy Roman Empire, and just as Enlightenment era philosophies guided the Americans and French to overthrow their monarchs, so too did they influence other people to desire their own rule.
A lot of historians consider revolutions to have a bit of a domino effect. The American revolution was a direct source of inspiration for the French revolution. French soldiers aiding the Colonial Army brought a lot of the Colonists ideas of democracy back with them to France (Ironic considering that one could argue that those ideas began in France with philosophes like Voltaire). After that, other people across Europe also tried to revolt against their monarchs in what are commonly known as the Revolutions of 1848. These revolutions included Austria, France (again), Germany, and Italy, as well as minor altercations, demonstrations, and reforms in Denmark, England, The Netherlands, and Belgium. Spain would even see a failed revolution just six years later in 1854.
You could probably also put some blame on Napoleon. Seeing yet another blood thirsty potential despot nearly take over half of Europe under another Empire probably led a number of people to question whether or not their own Emperors were actually that good.