these things... they happen all over this city.
For all my tl;dr in the depth section, it's hard to say why exactly I like this relationship so much. On paper, after all, it's an abusive, manipulative, and mutually self-destructive relationship: not exactly an easy sell. But I suppose that's what makes them so compelling. Seishirou/Subaru is like watching a trainwreck. Every time I reread the manga, I know what's going to happen. I know it's all going to go to hell in a handbasket. And yet I end up riding an emotional rollercoaster with Subaru every single time.
I really do think Tokyo Babylon is one of CLAMP's best works, largely because the first six volumes are all about establishing the backdrop of Tokyo and the message that no individual person can every truly understand what's in the heart of another — and then the seventh volume takes it to the inevitable conclusion in Subaru's relationship with Seishirou. I don't think X was executed as well (largely due to its pacing, which I find to be uneven at points), but the conclusion to Seishirou and Subaru's relationship is absolutely beautiful.
I guess I like tragedy. I certainly like other pairings with a certain element of futility. Not that I don't end up reading fanfiction where things go better — but I don't mind when those fics end up just as tragic as the original material, either. There's something to be said for an unhappy ending.