The Enlightenment's driving force was rationality and scientific progress. This premise helped to enhance the idea that individuals, through inquiry and rational thought, can help improve their world in ways that can benefit the larger social element. Emotions were seen as inhibitors to true progress and advancement. This is where the Romantics saw things quite different. As opposed to devaluing the subjective experience, the Romantics hoped to place primacy on it, believing that the individual narrative and sensation of emotional experience can help to reveal a sense of universal truth from the personalized notion of the good. In this, emotions are not to be overcome. Standardization tools such as science are perceived as the force of deadening, something to overcome.
In response to the cultural and political conditions in late 18th century Europe, Romanticism, or at least "romantic sensibilities," spread westward from Germany, France, and Italy into Britain, reshaping its literary and artistic landscape. No longer believing that the universe was a static machine, British Romantic authors and artists developed a "mystic ecstasy" in their works to emphasize a connection between the individual and a dynamic universe in which nature became a life-force of inspiration. British Romantic poets and painters saw flashes of universal insight; they became visionaries who departed from the temporal world in their works and set upon a sublime, almost "out-of-body" experience, to redefine the conceptions of beauty in art and nature. Artists believed they could create meaning in what they saw, that their minds were active agents upon the universe, that their visions were each unique and yet still connected to nature and their fellow man's.
I would say that the quick answer to this is that they are opposites because reason and emotion are opposites.
The major idea of the Enlightenment is that people should use their reason for everything. It is the idea that pretty much everything can be figured out through science and logic.
By contrast, the Romantics were very much into the idea of feelings and emotions. They liked things like nature because nature can give you intense feelings. They were into the supernatural because that gives you feelings that are intense as well.
You tagged this with "Frankenstein." I think you can see that as a Romantic argument against Enlightenment, scientific values (Victor's).
Comparison betweent th Enlightenment and Romanticism.
The Enlightenment or the age of reason as they refer to sometimes is considered as awakening from the long slumber of the dark ages in Europe. It's the fruit of Renaissance era. Also, it's the sparkle that gave the opportunity to the human mind and reason to spread. As a result, people began to think and become free from the authoritarian restraints of the church. In addition, Neoclassicism was a movement that appeared at that time and was marked by the classic writings (Greek mythology's references... ).The end of Enlightenment was due to competing ideas from several sources, the theories of skepticism came into direct conflict with the reason-based assertions of it (Enlightenment) and especially the French Revolution.
While Romanticism is a literary movement that came as a rebellion against the ideas Enlightenment; which depends only on reason, scientific methods, and skepticism's theories. It is clear that Romanticism is a call to imagination, spontaneity and freedom. In addition, it is characterized by its personal nature, its strong use of feelings, its abundant use of symbolism, and its exploration of nature and supernatural. Moreover, the appearance of the "I" or the individualism in the romantic writings is a new thing to literature at that time. Romantics was described as a unique and noncommon people also known as escapist from reality.
Romanticism is never opposite to enlightenment, it is a help and guide to super reasoning .For man by his inborn quality is imaginative .His to-day's imagination turns tomorrow's reality . This is applicable to science and social -science .Again feeling humanises man , it estalishes a bond of relation-ship , that leads to the platform of empathy .
The English romantic-movement is the outcome of renaissance . Discovery of new province , to see the unseen , to know the unknown , were the epitome of the essentials of romanticism .Blake , Wordsworth , Shelly , Byron , Godwin , etc , were the champions of romantic -ideals .
Reasoning varies from man to man .Dickens's The Hard Times , is a testimony of the evilest aspect of reasoning .So the term enlightenment may be taken for granted as the combined product of heart and head .We will be misled if we are taught to live on apparent discrimination .