love is a brittle madness
Twisted as it may be, there is no denying that there is love between Seishirou and Subaru. Sure, it's not normal or healthy, but it's there. This isn't a relationship where love conquers all — on the contrary, love is the worst thing that ever happened to them.
This section covers Seishirou's feelings for Subaru; you can find Subaru's feelings for Seishirou over here.
Let's start this off with a quote from the "Babel" chapter of Tokyo Babylon.
Seishirou: I don't know if this makes sense... but I think, in many instances, love simply isn't enough in this city. No matter how strongly you feel that love... it may just not be enough. After all, there's no place on Earth where strong feelings alone are automatically rewarded.
Subaru: Then why do people love at all? If love is useless, what use could we have for such an emotion?
Seishirou: Because we're lonely. As long as we can love, we can dream. Human beings aren't strong enough to live without their dreams. Let me tell you, I truly do love you, Subaru-kun. But I'm not stupid enough to demand the same feelings from you.
When I reread the series to revamp this site, I stopped and stared at this passage. I'd read it before, of course, but I don't think I'd ever realized the layers to it before. Anyone who's read the series more than once knows that Seishirou is lying, that he's wearing a mask, that he's the Sakurazukamori and that this is all part of his bet with Subaru. But the best lies have a note of truth to them, and I think there's a good chance that that's the case here.
Now, I don't know how much CLAMP had planned out before they wrote TB and X with regards to Seishirou's backstory, but the very last time we see him in X, we find out that the succession ritual for the Sakurazukamori is that they are killed by the one they love. Keeping that in mind, look at this quote again. Is this why he made the bet in the first place? Was it just that he was lonely?
Or is this, too, a lie?
In truth, it's hard to figure out when or how Seishirou fell in love with Subaru. We know he did, without question, because he says as much to Subaru when he dies and it's the entire point of the Sakurazukamori succession. Mostly, though, it's that Seishirou is a consummate liar and all I can really do is make guesses at his feelings.
I do think, though, that for Seishirou, love goes hand in hand with death. His very first kill was his mother, who told him that he would kill the one he loved most, who would in turn be the Sakurazukamori after him. So, to that end, he could not love Subaru without the knowledge that Subaru would be the one to kill him. Hokuto's spell ensured that it would happen no matter what Subaru thought about the whole thing, and honestly I think Seishirou used that to his advantage.
Hokuto: I'm putting my trust in you that you will never... set this spell into motion.
Seishirou: I'm not the kind of man you should put any faith in.
Hokuto: I know that. But... you still... mean so much to Subaru. So I want to trust you. And I want you to remember one thing. You may have committed sins you can never atone for... but no one is out of love's reach. Not even you...
Hokuto told Seishirou that her spell never needed to be used. She believed in him, and wanted him to be with Subaru. Seishirou knows that if he loves Subaru, he will be killed by him. For a long time, Seishirou believes that Subaru's wish is to kill him — after all, he did murder Subaru's beloved sister — and I think that in believing that, Seishirou allows himself to love Subaru. Of course, Subaru's wish is quite the opposite, but Seishirou is both too selfish and too shortsighted to realize it.
I do think Seishirou's actions are selfish throughout both series; I think to an extent the bet was simply a way for him to see if he had emotions, and though Subaru lost Seishirou ended up fixated on him anyway. He knew full well what would happen if Hokuto's spell activated, and took no steps to avoid it. Again, the whole concept of actually being with Subaru doesn't really register on Seishirou's radar; the way he was during the bet was just a mask, and to really love him could only end one way. Plus, you know, the fate of the world was at stake.
Fuuma goes into some detail about how Seishirou was, too, and I think he brings up some damn good points:
Subaru: There's nothing left for me.
Fuuma: Then do you want to die? Of course, if you do, his eye dies with you. Which means that suicide is out of the question. It seems to me that Sakurazuka Seishirou... was an incredibly selfish man. Don't you agree? I guess not. No one ever really understands someone else's desire.
Dying? Yeah, sure, Seishirou was fine with that. He figured that was the ultimate expression of love. But he also wanted Subaru to live at any cost, and he knew Subaru well enough to know that he wouldn't be able to let go so easily if he still had a piece of Seishirou to hold onto. In short: you are a selfish asshole, Seishirou.
Even so, he does tell Subaru he loved him. I wrote about this in more detail over here, and I think the series itself provides ample evidence of those feelings without having to see what he said (like, you know, the whole "only killed by the one you love" thing), but that doesn't tell us why. Furthermore, Subaru never knows about the Sakurazukamori succession (and with Seishirou dead, one wonders how the hell it'll be passed down from him, if at all). Personally, I think that telling Subaru how he felt was Seishirou's way of explaining himself. Because love and death were synonymous in his mind, I think he believed that all he had to do was tell Subaru he loved him, and that would be the end of it.
Unfortunately, the same wasn't true for Subaru. And so their story played out as a tragedy, and Subaru's wish was left unfulfilled.