Act II, scene vii introduces the Prince of Morocco. He has come to attempt to win Portia's hand by selecting, between three, the casket that holds the picture of Portia. If he chooses correctly, Portia will be his wife. This device has been set up by Portia's late father and not Portia. So, in essence, it does not matter whether Portia is impressed by any of her potential suitors or not. The caskets, as it were, will do the choosing.
Since Morocco knows that Portia has no say in her choosing, he shouldn't be assumed to be saying anything to impress or flatter Portia. There is no reason to believe that he is not speaking truthfully. And, what he speaks (a full 30 lines worth!) is all the highest of the high praise of Portia and his love for her. He says:
I do in birth deserve her, and in fortunes,
In graces and in qualities of breeding.
But more than these, in love I do deserve.
And later he goes on about how admired and desired she is around the world, and his reasoning leads him to choose the gold casket. He chooses wrongly and does not win Portia for his wife. And Portia, once he is gone, reveals her true feelings about Morocco (far from being impressed). She says:
A gentle riddance. ...
Let all of his complexion choose me so.
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