Answer:
The so-called flaw the duke identified in his last duchess was her heart, which was "too soon made glad, / Too easily impressed." He complains that she blushed no matter who complimented her, that "she liked whate'er / She looked on," and that everything and everyone seemed to impress her
The duke is upset that everything pleases the duchess. She is too innocent and too generous with her approval and too easily impressed. He later says that her holding any kindness in equal standing with his affection and his gifts “disgusts” him, though he refuses to “stoop” to tell the duchess why he is upset.
