Most significantly of all, there is an underlying darkness at the root of everything we have built. Outside the cities, beyond the blurring edges of our civilisation, at the mercy of the machine but not under its control, lies something that neither Marx nor Conrad, Caesar nor Hume, Thatcher nor Lenin ever really understood. Something that Western civilisation — which has set the terms for global civilisation—was never capable of understanding, because to understand it would be to undermine, fatally, the myth of that civilisation. Something upon which that thin crust of lava is balanced; which feeds the machine and all the people who run it, and which they have all trained themselves not to see. - Civilisation, The Dark Mountain.
There is a dark pessimistic streak of thought running through some romantic thinkers. Such romantics view nature not as benign matter to be molded by our will nor as an inexpressible force in perpetual flux which we can become part of through liberating our creative nature, but as a force hostile to human beings and to our designs. This darkness, this hostility, cannot be conceived or fully expressed by human language but it lurks behind our conscious mind and felt throughout history frustrating our dreams and our ambitions. Our civilizations will crumble, our works turn upon their makers, nature will reclaim her dominance over us for nature is not to be tamed.
It is the notion of uncontrollable will, not reason, dominating life, of unintended consequences driven by a reality hidden from our world of neat logic and material science. Some, like Schopenhauer view the universe as "simply a huge fathomless ocean of undirected will upon which we bob like a little boat with no direction, no possibility of really understanding the element in which we are, or directing our course upon it; and this is a huge powerful and ultimately hostile force, to resist which or even to come to terms with is never of the slightest use."*
This is why all our best efforts to improve have failed; why the French revolution collapsed into terror and the 20th century saw unprecedented slaughter. The explanations may make different forms: the wrath of God caused by the French embrace of atheistic materialism or the force of production or class warfare or secret powerful societies conspiring behind closed doors or the metaphysical forces of history or the limits of progress and impending environmental collapse. These explanations all share the same root, that of something larger and more powerful than human reason can grasp working through history or society or nature.
Artistically, this pessimism manifested itself in a new interest in exploring the dark side of the human psyche and sexuality. It comes through strongly in the nightmarish depictions of Goya and the dark erotica of Fuseli; the short stories of Tieck whose characters begin their tale by living happily but soon descent into destruction.
*Isaiah Berlin, The Roots of Romanticism.