Throughout the late eighteenth century and the early nineteenth century, most parts of Europe had a monarch who claimed to have the right to rule from God.
In opposition, the Enlighment thinkers challenged the legitimacy of those absolute monarchs and introduced the first democratic concepts during the last decades of the 18th century: social contract (the power of a state is held by the citizens who should transfer it to political representatives through suffrage), the division of powers (three branches of goverment: legislative, executive and judiciary, in order to prevent authoritarism), etc. These new principles directly threatened the pillars of the governments of the authoritarian kings, and brought reason to the political sphere.
When the legitimacy was questioned, the peace and stabiliy were broken and this is why the 19th century was denominated as the century of the revolutions in Europe.
