Okay, so I just went back and watched it, but didn’t hear an I before that particular line, just some of the pained grunts you’d expect from a guy with a magic burning sword to his throat. However, I definitely think Keith threw out the brother line first because he was hesitant to admit his real feelings–to actually say I love you. And I think there’s evidence as far back as season 2 that supports this, so let’s take a look at that:

If you’re going to talk about this scene, then I think it’s also worth looking back at the BOM. Holo Shiro keeps trying to convince Keith to give up the blade, and uses a number of guilt trips on him. The most prominenant one being, “You know exactly who you are, a paladin of Voltron. We’re all the family you need.” And when Keith says that, he stops for a moment to compose himself. We see him close his eyes, take a moment, then look back at Shiro and go for the brother line. Seriously, go back and listen to it. He says, “Shiro,” first, and then his voice cuts off as he takes that little pause before he finishes with the brother line. He hesitates. So when lots of LGBT people like myself talk about telling their close friends that they love them as just a brother or sister for fear of admitting their true feelings–it’s stuff like this that really hits home for us.

He says this becuase it runs in the same vein as what Shiro said, that they’re family–but still singles out Shiro above all that others. It’s not, yes you’re all my family, it’s that Shiro is. Again, Keith is definitely led along that line of thinking by holo Shiro’s comment about family. Before he delivers that line, Keith has to take a moment to compose himself for it. I think there was probably something else that he wanted to say, but he just wasn’t capable of expressing it at that time. Particularly when he was genuinely afraid of Shiro abandoning him. And then, to top if all off–before walking away, holo Shiro says, “Then you’ve chosen to be alone.” Because it’s painfully clear that the loss of Shiro, him above everything else, that sacrifice would be too much to bear. 

So, let’s look at how much Keith has grown by season 6, because it’s beautiful development. This is the BOM all over again, this is an alternate Shiro that tells Keith he’s literally worthless, that his parents abandoned him and he should too, that no one has ever cared for him. Kuron continually insists that no one has ever loved Keith, least of all Shiro. This is the kind of stuff that already broke Keith’s heart during the BOM, his darkest demons. But Keith remains ever resolute, never straying from his path. It doesn’t matter what Shiro says–and even after hearing the atrocity that “Shiro” had slaughtered their entire team, probably Keith’s mother along with them–even then, he still loves Shiro and feels Shiro is deserving of that love. 

But again, Keith is someone with deeply rooted abandonment issues. He pushes people away and builds up walls to protect himself. He never lets himself get too close to others for fear of rejection. And yet, as the show runners have said, Shiro is his one “exception.” There has always been this easy intimacy there between them, a bond that’s completely unique and unequivocal for Keith. And I think that, in a spur of the moment decision, empowered by all his tenacity and determination, but his willingness to do anything for Shiro’s sake–he was finally able to push himself to admit what he always wanted to say:

• Joaquim: “The one exception Keith allowed in his life in terms of expressing himself was probably with Shiro, because he felt the closest to him. And that situation was an extreme series of events. I think he was pleading with him. I think he was letting him know exactly where he stood, and why this shouldn’t be happening. And why he knew that there was still good in there…If you look at Keith going back to the original episode, where he comes out of the shack, and greets Shiro, that’s a very different Keith than the way he acted with all the rest of the Paladins. So I think with Shiro, in particular, his guard is down a bit and he’s able to express himself.”
  
• Lauren: “But don’t expect him to go telling anyone else he loves them.”
• Joaquim: “That’s right. He’s not handing out hugs and love hearts to everybody.” (source)
  

I think Keith being able to admit this heartfelt truth–to both himself and Shiro–is also very much a product of his two years in the time rift. He’s been given so much time to grow and mature, as well as to do some introspective thinking and really reflect on his feelings for Shiro, on what exactly their relationship means to him. Fans have also been pointing out all the clear parallels between Keith’s parents and sheith. And to some extent, I think Keith would be able to see that as well. Maybe by really seeing what a loving partnership looked like, he was able to start quantifying his own feels differently, and perhaps decide what he feels for Shiro isn’t strictly brotherly. 

Now, we all know that the brother line didn’t get any sort of response from Shiro. But Keith’s “I love you” yields this very real, visceral reaction for him. In scenes like this, you don’t start by referring to your relationship with someone using the most meaningful term and then work your way down. If you’re desperate and trying to convince someone they mean the world to you, you’re going to keep pushing further and further, until you actually manage to give the most honest and genuine, wholehearted admission that you can. 

For Keith, I love you is that. Shiro being like a brother isn’t what matters most about their relationship–might not even be an honest term for it–the fact that Keith loves him is what really counts. And ultimately, it was that and not the brother comment that broke through to Shiro. Because, deep down, Shiro knows that how Keith really feels–knows how much it means for him to say something like that out loud–and Shiro feels it too.