Instead of getting confused and angry about this, I will be more curious and open-minded. Sure it makes no sense, but that makes it all the more fun to theorize how the author will justify it!
I get that Mei has ptsd or whatever and wants to be needed by somebody to validate her apathetic existence but man I cannot dislike her more. All she does is act coldly and ignore her and make her feel terrible like she has to constantly apologize. If she had an inner monologue I would suspect we would see the inner turmoil and she would be much more likable. But 3 volumes of just guessing what she’s thinking has painted her to be a real piece of shit