In Shakespeare's famous play, Hamlet banters that "nothing is either good nor bad; only thinking makes it so." The perceptions of nature as preternatural rather than supernatural, the human soul as distrubed rather than stable, the mind as horrified rather than serene is Romanticism on its other side, a side that can be easily exhibited in the subjective point of view.
The Romantics romanticize what they see. In the 19th century, they look to the Industrial Revolution and they see children sweeping chimneys and the plight of the workers in the factories. Feeling outraged, they look for an alternative model of how to live and turn to nature. In nature, they see idyllic pastures and "dancing" daffodils (to quote Wordsworth). However, is this "romantic" view of nature realistic?
When we discuss "dark romantics," we see poets and writers that look to nature and find something horrifying, which they in turn romanticize in their work. A symbol drawn from the natural world is used to represent death and grief in Poe's famous poem, "The Raven." Rather than setting him free, nature is tormenting Poe's speaker to the point of madness. When Washington Irving sends Tom Walker into the woods in "The Devil and Tom Walker," he does not find an idyllic alternative to the corruption of the city. Instead, a devil is waiting for him.
Insofar as gothic writers reject the goals of the Romantics but use a similar class of symbols, differently interpreted and romanticized, to express that disagreement, they can be understood as "dark romantics."
Clearly Gothic fiction has many Romantic elements, and the two are very closely related. For example, you might find it interesting to think of how many Gothic novels also can be described as Romantic. Jane Eyre and Wuthering Heights, for example, both are described using these terms. Both Gothic and Romantic literature emphasise lived experience, but Gothic focuses on terror and supernatural forces, whilst Romantic literature focuses on inspiration, idealism and optimism. It is clear that Jane Eyre, for example, is Gothic in its focus on madness with Mrs. Rochester and the various (potential) supernatural happenings, but it is also deeply Romantic because of the idealism of Jane Eyre and the fairy tale ending we are given.
Gothic lit is also called the "dark side of Romance" (or better yet, of Romantic literature), because of the following:
Romantic literature is one known for the literary elements which it utilizes to create its atmosphere: An attention to human nature, crude realism, a salute to the past, the enticement of emotion, the need to recur to nostalgia, and the presentation of life AS IS, and not over glorified with excessive metaphor and allegory.
Meanwhile, its dark little brother Gothic literature, does the same with a twist.
While Romantics explore human nature through realism, the Goths explore the depths of human nature as far as the inevitability of fate, and the nature of men as weak and potentially bad beings.
Human nature is seen as an inevitable rendezvous with death and tragedy while the Romantics view its crudity without taking it to the Gothic extreme.
While the Romantics present crude realism, the Goths extended to supernaturally macabre realism.
The Romantics recur to the past, while the Gothics turn the past into nostalgia, longing, sadness, and loneliness as part of the inevitability of life.
The Romantics avoid excessive metaphor and focus of crude descriptions, while the Goths take it to the next level and use the saddest elements of life: disease, ruin, pain, psychological despair, and tragedy in lieu of metaphors. Plainly stated, the Romantics color life with reality while the Gothics color life with tragedy.
That is how Gothic lit is the dark side of Romanticism.
In many respects the Gothic novel or the Gothic genre explores the "dark side" of Romance because it strives to explore an aspect of the subjective that is not openly discussed. If we look at the Gothic exploration of love, for example, it is one where there is a considerable amount of brooding. There is some dark secret which possesses one or two lovers, something that keeps them from fully being able to immerse themselves in the love of another. It is this barrier and the striving against it while being cognizant of the impossibility of its overcoming that allows the Gothic novel to explore "the dark side of Romance." The Gothic novel is one where this hurdle can be emotional, supernatural, or simply destined to not be. It is within this light that Gothic novels explore a part of the Romance that involves pain, a type of longing, and the emotions normally associated with the down side of Romantic love.