The Narrative of the Life of Frederick Douglass, an American Slave fits within many of the central myths of the United States. The main one is that of the self-made man, who starts from nothing and by the force of his own will manages to "pull himself up by his bootstraps" and create a successful life for himself. We see this pattern not just in slave narratives, but also in many other iconic American stories such as The Autobiography of Benjamin Franklin or the stories of Horatio Alger.
In this prototypically American narrative, the protagonist, whether fictional or autobiographical is usually a loner, existing on the fringes of society. Despite being in some sort of subaltern or marginal role, this type of protagonist has a sense of difference from others in his (rarely her) situation, and usually struggles to obtain skills and education by means of extraordinary hard work and determination. Although benefactors usually appear at key points in the narrative, the notion of individualism and self-sufficiency is central to the story.
Thus the story of Douglass fits precisely into this narrative arc and character model of the American myth of the self-made man.