I don’t have time to even begin to list all the wonderful qualities of my (our) beloved Yuu, but let’s not forget that in regard to her understanding of what love is (or might be) she’s been pretty much an idiot ever since the very page.
Right, Yuu, wanting to be with someone all the time, enjoying kissing them, thinking about them all the time, trying to do what’s best for them, thinking about whether to touch them or not—that’s just regular friendship, not the “special” feet-lifted-off-the ground feeling that pop songs and romance manga have told you to expect. (Wait—kissing? Really? Also the touching? Hmm.)
Baka.
To be fair, Yuu was always someone who got overly invested into other people's problems. Example number 1, the softball club she joined just because she was asked to, even though she had no interest in it. Yet she stayed the entire time and was a valuable member. Example number 2, the student council. The list goes on. Yuu is someone who helps people without getting too invested into it emotionally. It's not strange that she at first assumed it was the same case with Touko.
She is very numb to the idea of love, because she has false expectations from Shoujo manga. Heck, even from her overly emotional surroundings and friends.
Yuu did realize something was off with her halfway through the manga, but rather than being stupid, she straight up denied herself the option to be in love with Touko, because of her promise. There's a huge distinction between denseness and denial. When she finally admitted (to herself) that she was in love, it was a big deal, because she knew it wouldn't end well and she did it against her better rational judgement. Instead of caring about herself though, she instead opted to fix Touko's self-loathing first.
The first time Yuu did something for herself despite knowing it would hurt her, she got completely destroyed, just because she had been in denial too long. Now her only defense against that pain is to return to her emotionally distanced state.
Sorry, didn't intend to ramble. The point is that (even if you said it tongue-in-cheek) I don't really agree that Yuu was or is acting like this because she is stupid/dense.