Analyze any quotation that Douglass gives about slavery.  To me, there seems to be a couple of reasons as to why Douglass has to employ emotionalism and a sense of appealing to sentiments in his writing against slavery.  Given the fact that he escaped from slavery, Douglass writes from a point of view that entitles him to be emotional and passionate about the need to eliminate slavery and what it does to human beings.  He lived it.  I would also suggest that appealing to the emotional sensibility of the reader of the time period was vitally important for Douglass because slavery was such an ingrained part of the American Experience, and was one that was embedded in so much of American society.  This requires him to adopt an emotional frame of reference regarding the need to eliminate slavery.