That's just her basic personality. I reckon it comes from being 300% too serious, which is also what other characters have noted. She holds herself to absurd standards, which she can't follow, making her unhappy. She also seems to worship nobility, which suggests she's from a family that has been serving nobles for 500 years and it's the only way of life they know. She looks no more friendly when she's talking to the duke himself, by the way. She merely has a pain in the ass personality. I'd say it's nothing personal.