Gothic: A word that originally referred to a Germanic tribe, the Goths, the term Gothic today is usually used to connote the medieval world in general and, in particular, a style of architecture that originated in France and that flourished during the Medieval Period, particularly during the thirteeth through the fifteenth centruies. Gothic buildings are characterized by a wealth of ornamental and intricate detail. Gothic architecture can seem flamboyant, mysterious, or even frightening. (Gothic cathedrals are often adorned with carvings of grotesque people, monsters, or devils knon as gargoyles.)
When applied to literature, Gothic is used to refer to a genre characterized by a general mood of decay, action that is dramatic and generally violent or otherwise disturbing, loves that are destructively passionate, and settings that are grandiose, if gloomy or bleak. The Gothic novel arose in late eighteenth-century England and remained popular into the nineteenth century throughout Europe and America; elements of Gothic novel and Gothic literature in general have persisted up to our own day. Some early forerunners of the Gothic genre are Samuel Taylor Coleridge, Henry James, Mary Shelley, Lord Byron and the Bronte sisters. Modern examples of Gothic authors would be Phyllis A. Whitney, Stephen King, and Anne Rice.
Popular culture continues to feature the gothic and gothic horror. "Goth Rock" enjoyed a vogue in the 1980's when musical groups shuch as The Cure forged a series of majestic, forboding rock albums awash with morbid sounds and themes. A more recent and extreme version of Goth Rock with a particularly provacative, even blashpemous, edge is purveyed by Marilyn Manson.
---paraphrased from The Bedford Glossary of Critical And Literary Terms
So as you see, the Goth culture today springs from these elements in art and literture. But my guess is that there are lots of Goths who don't know this, they're just trying to fit in with a group of people who think and act against social norms like they do. Which is also part of what it means to be Goth....