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Drake68655 A few reasons for all this really, in the "all of the above" kind.
1. Because love is weird, blind and ungrateful. As in, it is not impossible for the Duke to fall in love with the 'heroine' regardless of whether she cured him or not.
2. (Semi-Guess work) MC thinks the Duke is err "over relying" on her. Not sure what the right word is. As in, being the Duke's maid, he feels grateful/doesn't bother to communicate with others. Basically, she still sees him as a 'child'
3. I wouldn't call the MC dumb. I wouldn't call her smart either though. She is definitely one of those types that still thinks that life would go according to the story like some other series which would be more justified if she came from a different series.
[There was one series that I can't remember in NU. Its about a past life guy becoming a servant, improving his two master's (brother and sister) public approval/attitude so that they don't face execution and exile. He was successful with the public approval part but they still faced execution and exile because the world(literally) said so. The brainless numbskulls that "framed" the brother for various crimes THEN become smart(er), quite quickly though not immediately, and go "Why did I do that?" once they were released from their "roles". ]
So... maybe part dumb, part paranoid, unlikely-but-maybe forced (forced to be stupid by plotline , giving the MC a reason to go mad.) (I'd believe this theory if the Prince finds her and falls in love with her instead of the heroine.)