There is no passage, so it is impossible to answer this question. I apologise.
Answer: I think the is acting happy but internally she is sad.
Since Montag is not happy Mildrid cannnot be happy because other people around you affect your mood by their mood.
It might be aid that he is portrayed as a lazy person not only physically but morally. His heart was broken because he did not marry his true loved. So he became cynical, he did not care the state of his soul. he could not stop eating meat.
He might be considered a "Byronic " hero, he was against slavery and materialism.
