This began as more of a story and less of a character study. But then, it also was originally supposed to make more sense and be a little less (probably) confusing. Then it completely did what it wanted and this was the result. Well, I still kind of like it though. Here's hoping I'm not the only one.
Izaya loves all of humanity only and because they mean absolutely nothing to him. It's easy to love them, easy beyond belief to marvel at their behaviour and twist them in webs of lies to see what they'll do. Izaya can play with them as he wishes, and they can't ever force him to stop loving them no matter what they do, because they don't matter. He loves them as they kill, lie, cheat, and die – just as much as he loves those few honest, saintly, upstanding ones. And he can make them be all of these things, because he loves them and doesn't give a shit about them, and he's got a sharp mind and good eyes. They eat out of his hands like dumb animals; pigeons in a park and it would be disgusting if Izaya weren't so fond of them.
This holds true for all of them. From strangers in the street to his oldest friends and relatives, Izaya truly does love the human race. He feels affection that doesn't go away, and everything they do is a source of pleasure in some way or other. Certainly, some people – those he might call friends, and his family – elicit more emotion in him than others. But it's one-sided in a way: they can never really hurt him, because nothing they say or do will mean enough to bother him. Izaya's armor can't be broken because he doesn't have any – why should he need it, when their words and actions hurt just as much as downy feathers against his skin? Humanity has no idea how to fight, and that's just as charming as everything else about it.
There's only one exception to this rule of Izaya's, and perhaps it is the one that proves the rule. After all, it's not so much the fact that he hates Shizuo – it's just utterly impossible to love the man. If Izaya were anyone else, there'd be nothing special there, but Izaya truly loves everyone else, so this –
It wasn't immediate, unlike Shizuo's own hatred for him. Shizuo had just known, the moment their eyes met, that Izaya was his hated enemy and his path was set from there. Unlike Shizuo, who Shinra frequently postulated had increased access to such primal things as instinct as well as strength, Izaya simply had to wait for time to pass before he realized what was really going on. At the time Izaya had loved Shizuo as much as everyone else, had laughed at Shizuo's antics and had thoroughly enjoyed himself.
The process is difficult to explain; but somewhere along the road, sending mobs after Shizuo became less about observing his reaction and more because Izaya enjoyed watching the fights. Not out of any aesthetic appreciation for violence (Izaya enjoyed that as much as the next guy and no more, probably less; it was the motivations that did it for him), but because the prickling irritation he felt whenever Shizuo descended into a rage was still novel enough to be pleasant at the time. Eventually he figured out that he was experiencing aggravation, and he had to pin it on the fact that with Shizuo, there were no motivations.
Shizuo did not want to be particularly anything. He didn't try for anything very much either; he just was and that seriously bugged Izaya for some reason. Shizuo was strong, Shizuo had a quick temper, Shizuo hated fighting but still sent his every attacker to the hospital or at least seriously traumatized them. Shizuo didn't think anything through – and Izaya would have loved him despite that one, he would have because there were plenty of other people like that in the world. But Shizuo also operated off some sort of crazy instinct, some protozoan sensibility that allowed him to know instantly when he'd hate a person for the rest of his life, and let him claim he hated violence even as he was flinging objects that would murder their target if they connected.
It simply got to Izaya. How was never quite the question, nor even why: but instead Izaya constantly asked himself if. And every time he was just as astonished to realize that yes, he really did care what Shizuo thought and did. It wasn't as though he'd be upset if Shizuo insulted him, but Izaya felt it in every moment of their interaction, that somehow this person mattered.
To use an earlier metaphor: unlike most people, Izaya had never worn any emotional armor. He'd never needed it because fists felt like feathers. And then along came Shizuo – and perhaps this was all because of that monstrous strength, that damn unique neurological mix-up – whose fists were not only fists but battering rams. Izaya was completely unprepared for this, and every time, every time he was broken down, smashed into raw little bits that mattered.
And then, perhaps, sometimes it was only about control. Because ever since the very beginning, Shizuo had been so very predictable and yet… not, somehow, when it counted the most. He would fall for the simplest tricks and fly into delicious rages and then – all of a sudden – he'd quiet and look the picture of his name. He would know, without explanation, where Izaya was and what he was up to, and he would grin that furious grin and say, "I-za-yaaaaa…!"
And every time this happens, Izaya feels the same sort of shiver up his spine, something new and strange and entirely unpleasant. He can't understand Shizuo, but Shizuo clearly understands him, and he can't stand it. He wants to know more, to know everything, to be able to accurately predict Shizuo's reactions every time, but he can't and that's just insufferable. So he's got to make Shizuo suffer in return, got to, and he taunts him and destroys his life time and again – but still. Still, even after years of this, after so much effort and near-death encounters and fights too terrible to speak of, still –Izaya understands him no better than the moment they met.
Izaya can break anyone else, given time and the inclination to do so. He can worm his way in until he knows what they want better than they do, he can push them to the brink and make them actually want to step right off it into self-destruction. He can predict them and understand them and laugh at them and love them so much, but he'll never be able to do that with Shizuo. He'll never be able to stop trying either, nor even to forget for a single second, because this is Shizuo.
The hatred Izaya feels for Shizuo can never be summed up in a single sentence. It can't be explained without this sort of complicated monologue, this self-contradictory meander through Izaya's very being. This hate is more than words, more than actions, more even than hate itself – sometimes Izaya feels like his hatred of Shizuo is all that exists, all that matters. Izaya is terrified of ceasing to exist, and willing to do anything to keep surviving, and somehow he has this horrible certainty that without Shizuo, he will do exactly that. Shizuo is all that keeps him human, or even sane, and Izaya resents him for it so much that he can never stop trying to destroy him. And yet, Izaya knows that no matter how he talks or plans, he will never succeed, because Shizuo is a monster. Shizuo is not capable of dying, Izaya believes that somehow despite knowing so much about Shizuo's vulnerabilities. Perhaps it's the child in Izaya, that cries for Shizuo's destruction and yet remains convinced it's impossible. Perhaps it's still the child in Izaya, that is a little relieved, a little delighted, and a little tremendously sad every time Shizuo fails to die, or break, or disappear.
And – well – this is only sometimes, really. It's not like Izaya thinks this way all the time. Not really. He's not so sad and horrified and angry as all that – Izaya is almost always in a good mood, after all. He can even think of Shizuo without fury, can smile and feel almost nostalgic and wanting for something he's not sure even ever existed. He can laugh and taunt and tease and enjoy it all, because Izaya knows he can do this forever and while it matters, will always matter, it's not going to end.
Everyone else might. Everyone else is perfectly welcome to do whatever the hell they want, or what Izaya wants them to do, until they drop dead of it. He'll still love them exactly the same, and he'll be as unaffected as if they never existed in the first place. But Shizuo is the exception and always will be; Shizuo is never going to make sense or do what Izaya wants – not without those moments in which he defies every law and proves himself a monster yet again and demonstrates that he knows Izaya better than anyone else ever could. Shizuo is always going to matter and on the battlefield to keep existing, the struggle for Valhalla that Izaya has already begun to weave throughout Ikebukuro, there are going to be two chosen survivors and no more.
Celty's head will wake for Izaya, and grant him eternal existence, which is all he needs, all he wants. Izaya will be the only human to get this treatment, the chosen warrior, but there'll be a monster too, because there can't not be – Shizuo will survive even death and will continue existing to torment Izaya forever.
Izaya clings to this knowledge – for it is knowledge, he knows it as surely as he knows he's breathing – like it's the only thing that exists in the world, and it might as well be. And Izaya can't stand Shizuo, hates him so much and wants him gone.
…But Shizuo will never be gone. He's a monster, a fool, and perhaps the way Izaya feels about that is love in a different sense. Perhaps Izaya really just hates humanity, despises them and loves Shizuo with a furor he can't even bear, a love that is need more than want, desperation more than like, and never, ever capable of stopping. Perhaps that's the way things really are, but the reverse is easier and it's not like the switch of words changes the reality.
So, although it's difficult, it's also impossible for Izaya to resist, and he does it with all he ever is: Izaya hates Shizuo.