ha, accidentally stumbled upon my notes written soon after the 2nd volume got published. For whatever reason I failed to post them here back then. But I'm surprised they actually turned out to be quite to the point. You may not agree of course. Still, I guess this also shows how consistent the story has been so far.
The way I see the story, Touko faces difficulties with receiving love, but is willing to give as much as she can. And Yuu has difficulties with giving love while being exceptionally easy on the receiving side, not rejecting Touko in any fashion. So they work kind of like role models for each other - Yuu is perfect to teach how to receive love and Touko shows Yuu how to share love. So to me it's inevitable that through some drama hardships they will converge to a couple.
Yuu doesn't know how to love. But why? It's very hard for me to imagine. How could she never have fallen in love before? At first I thought, well, if she's gay, then of course it explains why she's never truly fallen for guys. But then she should've fallen for girls, right? And yet she hasn't. And I really find it hard to believe that she's never had anyone around her to make her heart skip a single beat. Therefore, I think that Yuu has simply been always conscious and afraid of her sexuality, shutting herself in the closet from any romantic sprouts till someone safe, openly gay for her, like Touko, pops up, finds her there and brings her out.
And why would Yuu's personality become like that? In chapter 04 we saw her father saying how he would've been knocked down if Yuu turned out to be a lesbian. Not that he would be mad at her or would try to prevent her from going that route, but clearly Yuu felt it'd be a severe blow to him. And she doesn't look like the person to hurt her family.
This would explain, to me at least, why she'd learn to hide and ignore her actual feelings. And given her sister was the one who came up with a girlfriend idea, even if as a joke, I would guess that Yuu could have given some reasons for that in the past too. And by saying that I imply that Yuu might have been used to ignoring her feelings for quite a while now. So this would explain why Yuu claims she has never really felt anything for anyone, boys or girls. This could be what rendered her being unable to give love.
As for Touko, her sister died when Touko was 7 years old. It seems this made her feel lonely and abandoned. All the attention was drawn from her to how great her sister was. She wanted the world's love back, so she thought if she were as good as her sister, everybody would love her too. Only to learn that all she got in return were love crushes and flattery. Well, maybe some positive praise too. But none made her feel less lonely apparently. I don't think she quite understands the following yet, but once the play by the student council is through, the goal replacing her dead sister will be achieved, and Touko will likely find herself all empty, blank, and lonely again. Because she still has never truly responded to any of the feelings people have thrown upon her, good or bad. Which is why keeping the love so one-sided, rejecting to receive love, even if she really likes Yuu, Touko may really hurt Yuu a lot when the time comes.