Just because she did not follow him as he demanded of his duchess. Leaves one asking if he'd ever told his wife how he felt, did
he ever indicate that he ended up feeling so murderous due to her actions? One would hope that as a married couple and all that the Duke would speak about her acts of insolence with his Duchess. The duke attributes to his social status the inability to express his preferences to his wife. Even if she would tolerate a criticism or guidance. Not unexpectedly, a man with such arrogance will not lower himself in order to say her his will. She should agree with him all the time, he assumes. The duke believes she should be able to read his mind
There is some indication in this expression of the Duke's grievance as to why he did not actually express his displeasure to the Duchess. He is a proud man and feels that to criticize such "trifling" misbehavior on the part of the Duchess would be "some stooping; and I choose / Never to stoop." The Duke realizes that his complaints, while valid to him, might seem petty to others and that if he should object, it would belittle him in the eyes of the Duchess and others.
