People always sought to justify their relationship, but they couldn't care less. [Seto x Yami]
If anyone else had to put it down to something, they would have said it was mutual intelligence. They were gifted with sharp minds, and both knew how to use them, and it was a widely accepted belief that smart people liked other smart people. And so, that was what the rest of the world put it down to.
Close friends, along with that reasoning, would have added that it was only natural after all that they'd shared, what, with the repeated saving of the world and the constant rivalry and the nearly being killed... Again, it was just one of those things. People labeled it as they saw fit.
Family might have considered those reasons, and probably even held their own, but knew better than to share them. Sugoroku and Yugi and Mokuba were the same as each other, in that respect.
But if you were to coax an answer out of the men themselves... Well, you wouldn't be able to. But if you could, it would be far simpler. They are both proud men, and born leaders. They bonded over the silently furious love of and need to protect their siblings-or Yugi, in Yami's case, and that's close enough. Bonded over lullabies that have been etched into their minds that they automatically recite when the thunder growls. Over the silences that are used more frequently than words, and understood better. Over chocolate ice cream and animated movies. And shared drives and devotions and duties, and the want for something more than these... In short, they understand each other, nearly perfectly, and that is more than anyone could ask for. So they cling to it. And they slowly fall further and further into the elusive and much sort-for thing known only as love.
But you wouldn't be able to coax an answer out of them. In fact, I'm not sure if they know the answers themselves. Though I don't understand why not. You need just look at them, whether they're sleeping side by side on the couch after another all-nighter, or wandering through the local park discussing Duel Monsters and the latest flaw in the programming, walking no less than a foot apart as to keep up public appearances, of course. You just need look at them, to see how stupidly in love they are.
But since when have reasons mattered? Love is love, and no one who matters to them cares about anything else.
...Including Joey, even if he still thinks Kaiba's a jerk and that Yami could do better if he wanted.