Sir, ’twas not Her husband’s presence only, called that spot Of joy into the Duchess’ cheek (13-15)
The Duke is offended that the Duchess would take pleasure in anything other than him. Notice that the way she shows her pleasure is involuntary, (i.e., a blush counts as showing pleasure), but the Duke describes it as though it were a stain or taint, a "spot of joy."
perhaps Frà Pandolf chanced to say, "Her mantle laps Over my lady’s wrist too much," or "Paint Must never hope to reproduce the faint Half-flush that dies along her throat" (15-19)
The Duke’s jealous fantasies are very elaborate – he’s imagined in detail the kind of compliments that the painter might have paid to the Duchess, and the coy way that she might have responded. It’s important to remember that, as far as we know, this could all be in his head. There’s no evidence in the poem that the painter said these things or that the Duchess blushed in response.
such stuff Was courtesy, she thought, and cause enough For calling up that spot of joy. (19-21)
The Duke seems to believe that the Duchess chooses to blush or react to compliments and gifts. He describes her as "calling up" her blushes, instead of experiencing them as an involuntary reaction. As readers, we know that she probably isn’t blushing intentionally, and the Duke’s jealousy is illogical.
She had A heart – how shall I say? – too soon made glad. Too easily impressed: she liked whate’er She looked on, and her looks went everywhere. (21-24)
When was the last time you heard someone complain because their spouse found joy and pleasure in too many things? "Man, I can’t stand my wife, she’s happy all the time," you might imagine the Duke saying. The Duke fantasizes that this pleasure in the world implies that his wife is promiscuous – a stretch, to say the least.
Oh sir, she smiled, no doubt, Whene’er I passed her; but who passed without Much the same smile? (43-5)
The Duke thinks of kindness as less valuable if it isn’t selective. As he portrays her, the Duchess is a kind and attentive wife to him, but that means less, in his mind, because she’s kind and attentive to everyone. He wants her to save all her affection for him alone – classic controlling abusive husband stuff.