Why were slaves like Douglass an offense to their mistress

From the text:

I know of such cases; and it is worthy of remark that such slaves invariably suffer greater hardships, and have more to contend with, than others. They are, in the first place, a constant offence to their mistress. She is ever disposed to find fault with them; they can seldom do any thing to please her; she is never better pleased than when she sees them under the lash, especially when she suspects her husband of showing to his mulatto children favors which he withholds from his black slaves.