The influential narrative of Frederick Douglass is an awe-inspiring description of his struggle and fight for freedom against the prejudiced, brutal, oppressive Southern institution of slavery. His motivation to write about his difficult life as a slave was to both inform the American public about the debased, malevolent nature of slavery and to humanize the slaves negatively affected by the Southern institution. Throughout his narrative, Frederick depicts the horrors of slavery by exposing the slave masters use of physical and psychological abuse. Frederick Douglass's vivid descriptions of his emotions and thoughts also humanize the enslaved African Americans. At the time, slaves were considered less than human and thought of as emotionally shallow, unintelligent beings. Douglass shatters these prevalent false stereotypes by articulately depicting his emotional depth, talents, and critical thinking skills throughout the narrative. He presents himself as an intelligent, rational individual with integrity and an unrelenting spirit. Frederick Douglass not only exposes slavery as a perverted, horrific institution, he also creates sympathy for enslaved African Americans by portraying them as equal yet oppressed human beings. Douglass's use of pathos persuades readers to empathize with the plight of the enslaved African Americans. One of the most moving scenes of his narrative is when Douglass laments his current status as a slave by writing,

O, why was I born a man, of whom to make a brute! The glad ship is gone; she hides in the dim distance. I am left in the hottest hell of unending slavery. O God, save me! God, deliver me! Let me be free! Is there any  God? Why am I a slave? I will run away. I will not stand it. Get caught, or get clear, I’ll try it. I had as well die with ague as the fever. I have only one life to lose. I had as well be killed running as die standing (74).

Frederick Douglass's narrative challenged Americans to view slavery from a different perspective by revealing its true nature while simultaneously depicting African American slaves in a positive light.

Frederick Douglass's purpose in writing his autobiography was not only to show the way in which slavery degraded slaves but also to show the way the institution of slavery degraded slave masters. He shows this in part through the character of Sophia Auld, who is a kindly woman when Frederick Douglass first arrives at her house as a boy. She even attempts to teach him to read, but her husband criticizes her for doing so, saying that teaching slaves to read will make them unfit for slavery. She stops teaching young Frederick Douglass to read and becomes a hateful woman. Douglass emphasizes that Mrs. Auld, who has never held slaves before, has been turned evil by her association with slave ownership. His purpose in drawing this and other portraits of cruel slaveowners in the book is to show northerners how evil slavery is and how it degrades white people as well as slaves.

As stated in the previous answer, Douglass's purpose in writing this book was to reveal the evils of slavery to the wider public. To this end, Douglass pulls absolutely no punches in his depiction of the horrors of slavery. Not only does he speak about what he himself went through, he provides horrifyingly graphic details of the sheer sadistic cruelty of some overseers towards other slaves, who would flog them almost to death, and so on (the maltreatment of Aunt Hester is one particularly lurid example). Douglass himself was finally able to escape this miserable life, but his account remembers those who did not.

Throughout the book, Douglass never loses his angry, bitter, accusative tone. Indeed, the work comes across as sensationalist, but this was not fiction: it was grounded in actual experience. The book was intended  - and proved - to be a shocking eye-opener for many readers who previously may have been only hazily aware of the extent of the suffering of many slaves.  It became a clarion call for the Abolitionist movement, and Douglass himself a highly respected activist and speaker for the cause.

The book, then, undeniably serves a distinct political and polemical purpose, but it can also be taken as a personal attempt by Douglas to try and work through his grim experiences as a former slave.

Simply stated, Douglass was attempting to expose the horrors of slavery to a large reading public. His book was a highly political document, intended to foster opposition to slavery among educated Northerners. The abolitionist movement was growing, but as persuasive as the writings and speeches of such firebrands as William Lloyd Garrison and Wendell Phillips could be for some Northerners, they were no substitute for first-hand accounts, written by former slaves themselves. Douglass's Narrative fits into a genre of slave narratives that included accounts by Olaudah Equiano, Venture Smith, Harriet Jacobs, and many others. Douglass was as successful as any in exposing the psychological, as well as the physical effects of slavery on the enslaved.