Douglass describes the songs that the slaves sing carefully because he wants the reader to know the real meaning and emotion behind their singing. The slave owners thought that since they were singing the slaves must be happy so Douglass explains how they weren't singing out of joy.
*I would've put some quotes in there and some more info to back up my argument but I haven't read the book in awhile so I vaguely remember the specific reason why (but it should sound something like that) and I don't have the book in front of me. Hopefully that was enough for you to go off of. :D *
