At this point, Yui and Yukino are both fully aware of each other's feelings for Hachiman. Yui herself finds the resolve to make a decision, which happens to be a very serious one: giving up on Hachiman for Yukino's sake. To do this she asks both Yukino and Hachiman out on a date. At the surface it seems like a simple gathering of friends, but at the heart it's something much more. Yui's ultimate goal for this "date" is to force Yukino to confront her feelings for Hachiman, by pressuring her with the indirect (though direct as indirect can be) admittance of her own feelings. She seeks to use her own feelings as a weapon to force Yukino to make a decision. I think in the end, she saw the same exact thing Haruno saw: a love triangle spiraling towards disaster. As Yui starts off by playing wingman, we see how difficult this decision is for her. The bittersweet expressions on her face, coupled with Yui's rendition of Hello Alone playing in the background (even as a Yukino supporter, that was incredibly depressing), show just how much it kills her on the inside. For the happiness of her two best friends in the world, she's actually willing to throw her own feelings away. And when the time comes to set her grand scheme into action, it almost goes just the way she planned. Visibly trembling from head to toe, she puts her own feelings out in the open by giving Hachiman her Valentine's Day cookies, which is essentially a challenge to Yukino to confront her own feelings right then and there. This shocks Yukino and puts her at a lost for words. As Yui solemnly awaits Yukino's answer, it's apparent that she's prepared to have her heart broken. I think if Yukino had openly admitted her feelings right then and there, Yui would have chosen to back out for good.
It's pretty clear that the mind of Yukino is a mess right now. Though she's coming to grips with her feelings for Hachiman, she is not yet prepared to take the next step in her pursuit, as evident in her being unable to give him her Valentine's Day cookies not just once, but twice. When Yukino realized Yui has feelings for Hachiman, she may have started to feel guilty about her own feelings, which made it all the more difficult for her to confront them. She also sees through Yui's grand plan to extent: she notices that Yui is trying to play match-maker, which makes her visibly uncomfortable. At one point, she even leaves Yui and Hachiman alone out of conflicting feelings, something Yui picks up on. But despite her keen observations, she wasn't at all ready for the stunt Yui pulled at the end of the "date". In a panic, she frantically tries to find an answer for Yui, and her thoughts become a jumbled mess. While Yukino may be conscious of her feelings for Hachiman, she's also scared and confused. Her problem is that she's thinking of everything that could possibly go wrong if she were to actively pursue her feelings. She's scared that she might lose her best friend in Yui. She's scared that Hachiman might not feel the same way. She's scared that the she might lose the club. She's scared of getting burned again by people she holds dear. Despite all of this, it seems she still found the resolve to give Yui an answer. But, before we can hear it, Hachiman cuts her off. After being saved by the 8, she elects to finally make a long overdue request to Hachiman and Yui. And again, before we can hear it, it ends on one of the most gut-wrenching cliffhangers ever.
I think the reasoning behind Hachiman stopping Yui's plan is pretty simple: what she was doing wasn't genuine. At the very least, Hachiman could see that Yui was forcing herself to do something that would only end in self-inflicted pain. Hachiman, more than anybody, could relate to this situation and put himself in her shoes, because he's self-sacrificed more than anybody else. He knows that what she's doing isn't the right thing to do. Whatever the future has in store for him, Hachiman doesn't want the foundation of it to be built on deceit. I'm not even sure how Hachiman would answer either of the girls at this point. He definitely has special feelings for Yukino, but I wouldn't say that he doesn't feel something for Yui as well. At the very least, he doesn't want to see Yui suffer like the way she was making herself suffer this episode. I also think he'd prefer Yukino express her feelings on her own terms instead of being pressured into it. He even says outright that he views Yui as a kind girl, and Yukino as a strong one. Seeing them both interact like that at the end contradicted the ideal images he had for them. In the end he was really looking out for them both. Even so, to say that the guy has a lot of thinking to do would be an incredible understatement. But at least now with that cliffhanger, he has all the time in the world to make his own decision.
Thank you Wataru Watari, Studio Feel, and all of the wonderful cast (Nao-bou, Eguchi, and Hayamin especially) for putting on a damn good show. If you haven't already guessed, this is my Anime of the Season for Spring 2015, and might well even be my Anime of the Year for 2015. Even though ending it like that was almost cruel, I can at least be a little more optimistic than I was before about the possibility of a future resolution being adapted in some way. (because come on, you CAN'T just end it like that, RIGHT? I'll even take an OVA). So with that being said, I think I speak for all of us when I say: SEASON 3 WHEN