@etriuswimbleton there's 2 basic motivation for personality change:
1) You hate yourself
2) You want to be better version of yourself
The difference between those two is the existence of a goal. For no.2) it is a idolizing mechanism. When you have a goal as your "ideal self", usually an idol or inspiration, you will be motivated to change. Doing anything by your own volition to be the ideal version of you. Like a proper traveler, they have a map, goal/destination, budget, and plan to go there. They take the step to go out not because they HAVE to go out, but because they WANT to go out, to a better place, and they know how to go there.
no. 1) it is simply just escapism coping method. You want to be something, someone, that is not you. You dont care what kind or what version it is, just not being yourself right now. Usually you will just pick the one which will be liked by the most in your society/people around you, the normal & general version. Dont have any ideal self-image to achieve. Just like a lost traveler-slash-adventurer, they dont have any destination/goal. Just walk somewhere, as long as it is not their home, and just try to be play along with the locals. Become the person that wont be hated by them, and try to like what they like. Usually this is type of escapism is NOT happen by inner will, just by pressure of environment or society around them that make them cant stand that environment or being themselves in that environment that flatly reject them.
It is the same with how Shizuka's parent keep telling her to speak loudly. She, the inner will of Shizuka-chan, is okay by being mute & use book to speak, but others pressure that keep telling her to change is what make her stress & pressuring her to change.
Making Karane become meek instantly is the equivalent of making Shizuka become a loose mouthed gal. They cant do it, and by pressing them will just make them hate herself and even if it is possibly happen, it is just become a farce. Not a true nature/personality that they develop themself.
That is what Rentarou dont want to happen. Karane is Karane. If she, by her own volition & will, is really want to change, because she idolizing & imagining herself become "better" it is okay. He will try to learn to like this "new" Karane.
But if she do it because she just hate herself, or being pressured either by atmosphere or society that prefer a meek girl in a group...He wont accept it.