Why did Charles Dickens Write ATTC?

My best hypothesis for Charles Dickens' purpose is this: a comparason of a couple of tumultuous lives based in two of Europe's most significant and populous cities would provide Dickens' a proper medium to explore the light and dark sides of humanity. As we learned from our in-class lectures, Dickens' was consumed by explicating the dark (evil) and light (good) characters that existed in all people, and even more importantly in himself. The different settings provided two different atmospheres for the nature of a human being to be unraveled, allowing the characters to act as models for different parts of society and making his study more comprehensive. The paradox of the novel brings the two cities together to provide the comparison.