It seems obvious to us that slavery was awful. But part of Douglass's journey, believe it or not, is his discovery of what slavery really is. When he is young, he doesn't really understand what it means to be a slave; he only starts to get it when he sees his Aunt Hester being whipped by his master. His real introduction to suffering occurs when he goes to work for Covey, and it's here that he learns to overcome suffering. When he vows to die rather than let himself be whipped again, he gains the strength of will he will eventually need for his journey north to freedom.
Before Douglass can become free, he has to learn the truth about slavery. Since the truth can only be found in great pain and loss, he has to survive through tremendous suffering before he can really become free. In other words, the path to freedom is through suffering.