I don't hate the "I don't understand love" concept (heck, YagaKimi is one of my all-time favourite manga), but the way it's done here feels so out of place and more like the author is trying to write himself out of a corner with these two rather than a conclusion that he's arrived at organically.
Not that this is a problem with this manga specifically, many serialised stories run into this type of roadblock sooner or later.
It sorta does make sense with her character - she's been watching Anjou and Toyoda being all lovey dovey with their boyfriends, and watching them (in her mind) drift further away from her.
But she also sees them happy, even without her, and that's part of the ongoing tension that Chita brings to the narrative. She fears change, wants things to stay the same and for her friends to always be by her side, but she doesn't get why they want to be with their lovers, and why they want to keep moving forward when it scares her so much.
At the same time, she knows that Tokio has feelings for her, but whatever she perceives from him, she herself doesn't feel--only that he's also important to her, but in ways she can't square beyond 'childhood friend' or 'foodie partner'. But the
kicker for her, is now Tokio is getting attention from others, and she now sees the same pattern potentially playing out that did with Anjou & Toyoda--and Tokio might leave her behind, too.
But she also knows he's told her he won't, but that uncertainly over not really
getting it--getting his "love"--means that she's got that stress of not knowing, and being afraid of her last close person abandoning her (again, in her mind).
And so her "I don't understand love" is born of all of that: her fear of change, her ignorance of what love & romance is and how it connects people, and how all of that affects her and how it could make things "not the same" in ways she can't comfortably predict.
Chita doesn't understand love because she's never experienced it in the way she's seen others experience it, but she does know that they changed, and that scares her--but she's realized that Tokio is her last chance, and if she wants to keep him around, she needs to confront this at last and sort it out.
At least, that's my read on it.