Izaya is a strategist, while Shizuo is hot-tempered. You just need to take a look to understand that the both of them are polar opposites. Even when talking about their appearances, they are totally different. While Shizuo is tall, muscular, and blonde, Izaya is small, and appears to be frail.

Izaya is a very calm person. He has the incredible capacity to remain cool whenever anything happens, and detach himself from the situation. He has the power to coerce people into acting the way he wants, and enjoys watching his plans going the way he wants from afar. We could say he doesn't dirty his hands, and yet his hands are dirty.

On the exact opposite side, Shizuo is one hot-tempered fellow! He just loses his mind every single time he sees Izaya. It doesn't matter if Izaya doesn't do a single thing to provoke him, when Shizuo sees Izaya, he simply loses it. Shizuo is all but a strategist, so he just takes heavy objects and throw them towards Izaya, who – thanks to his agility – manages to escape!

Their relationship has always been like this. From high school onward they hated each other, and have been at each other's throats. Their rivalry looks pretty much unjustified, but at a better glance we can see why their hate toward each other keeps on growing.

Izaya says very often that he likes people. The truth is that Izaya, who is a bit of a sadist, likes to put people in difficult situations, and see how they react. Shizuo, instead, would like to be considered as a normal human being, and not as an emblem of violence. In fact, he says very often that he hates violence – a keen way to imply that he hates himself.

So what is the flip side of these two characters? Well, it's easily said. They both just want to befriend someone. They both want someone who loves them just the way they are. The fact is that Izaya puts a lot of effort to draw people closer to him, but he acts the wrong way, so he ends up being left alone. On the other side, Shizuo, who is violent and always angry, doesn't do anything to draw people closer to him, and yet he manages to make friends without effort. Izaya is jealous about this. While Shizuo is jealous of Izaya being considered normal, and being able not to lose his mind easily, in the end, they would just like to be the other!

As we said up above, they are not too honest to admit that in the end the other party is exactly what they would like to be. If there's a thing they share, then, it's their way of dealing with other people. Or well, their way of not being able to deal properly with other people.

Shizuo gradually isolated himself after realizing that his strength could hurt the people he loved the most; so he is a solitary guy, wandering the streets of Ikebukuro and resigning himself to loneliness. What he cannot see clearly, though, is that there are a lot of people who like him just the way he is.

Izaya, who puts a lot of effort to draw people closer, instead, acts in the worst way possible. Maybe he thinks that is the right way, but people are scared of him. He is a strategist, and an underground information broker, after all. So although he says that he likes humanity, his kind of like is very distorted. As we said before, he likes how human nature can change in various situations. What he really likes is to see how people act when stressed, and how they react to a crisis.

What we see, then, is a couple of guys who just can't deal properly with other people, and keep on acting in the only ways possible for them.

Despite their hate, their rivalry, their rage, and their differences, they keep ending up facing each other every time there's a chance. Let's say it clearly; Shizuo and Izaya hate each other, and yet there's no Shizuo without Izaya! It's clear as daylight that the only person who is able to make Shizuo go mad with his only presence is Izaya. At the same time, the person Izaya loves teasing the most is Shizuo.

They hate each other, but they come to each other's throat whenever there's even a little chance to do so. What can we call their relationship? It's not friendship, and it's obviously not even bromance. It's something very near to obsession. In fact, despite his hatred towards Izaya, Shizuo tends to think about him more than necessary. The same goes for Izaya, who is always prone to find new ways to make Shizuo lose his marbles. What's more, even when it's one of the rare moments when Izaya is simply breathing, his sight for Shizuo is just too much to take, and so he just starts fighting with him. Izaya is one fast fellow, so he can slip away from Shizuo very fast, and so they keep on doing this Cat & Mouse game endlessly. With their way of acting, it was only natural for some female viewers to catch a glimpse of Boys Love between them!

They say “opposites attract”, right? Well, it's exactly the case with Shizuo and Izaya! We already said that Shizuo is impulsive, and fast at using violence. What we didn't say, though, is that between Shizuo and Izaya, the most aggressive one is Izaya, for as paradoxical as it can be. Shizuo could keep living peacefully, without having to deal with Izaya. On the other side, though, Izaya hates him so much that he has to bug him every time.

As we said up above, they are on opposite sides. Possibly, this is what draws them so close. They have this sort of unspoken admiration towards the other that makes them hate the other. Neither Shizuo, nor Izaya would ever admit it, but both of them crave for something that the other one has. For Shizuo, it is the way Izaya manages to be so calm in spite of the situation, while for Izaya, it is the way Shizuo – in spite of being as violent as a wild animal – manages to draw people close to him, without even trying.

As far as we know, the contrary of love is not hate, but indifference. These two fellows are all but indifferent to the other... More like they are obsessed with the other, and what's more, each and every step they take, is closely checked by the other. It's not a stretch to say that if Durarara!! would've been a shounen-ai anime, they'd have ended up together in the end!