The Romantics believed in the Classical concept of nature. The natural world is like the Garden of Eden: a paradise, uncorrupted by mankind, the perfect synthesis of the divine, human, and organic. Sin causes man to be cast of this Utopia; corrupt human nature leads to exile form paradise; knowledge leads to rebellion.
So, the Romantics pit humans against nature as an overriding external conflict, and they use knowledge against their characters as a sign of humankind's irreconcilable fallen state from nature. Mental illness and madness are the most obvious signs to show the Fall.
Also, Romantics are extremists. They esteem the sublime (the divine, the most beautiful) in nature and the Gothic (the most evil, the darkest) in human nature. Whereas the natural world is a Utopia, human nature is Dystopia.
Many Romantic characters are extremely loyal and then extremely cruel and then insane (the Macbeths). Or, they are extremely and passionately in love and then extremely vengeful and cruel (Heathcliff). Or, they are threats at night and dormant during the day (Dracula). In this way, the Romantics use human emotions as barometers of the sprits of the age: rebellion, irrationality, and passion.