Light doesn't know what happiness is.
His entire life, he's been focused on being the model son, the perfect student, the best at everything. Flawless, beyond reproach in every possible way. He has always been this way. It's just who Yagami Light is.
But he has never quite been happy. That is not to say he has been unhappy, because he is extremely intelligent (though perhaps not always smart) and knows that there are degrees to everything, even such ephemeral things as happiness.
Sometimes he lies awake in the dark, on those nights that Ryuuzaki decides are for sleeping, listening to Ryuuzaki's almost-silent breathing, knowing that Ryuuzaki is awake and knows that Light is awake. And if they're both awake, why bother maintaining that ridiculous pretence of sleep? But they always do and Light, in those odd pre-dawn hours floating somewhere that is neither waking nor sleep, ponders happiness.
One night he wakes suddenly from a dream of shackles and cold floors and metal bars and no way out, gasping for breath as though drowning, and Ryuuzaki, who is really L, who is Denuve, who must be a dozen other people who aren't even real, takes his hand. They twine their fingers together and Light presses his face into the cool silk of the pillowcase and cries without tears for something he can't quite name and doesn't understand. L says nothing, but his grip tightens and Light squeezes back and finally succumbs to a sleep that brings no other dreams.
In the morning neither of them mention it, but the next time they lie down to sleep their hands meet beneath the sheets and they hold on tightly. Hands are fragile things, but somehow enough (just barely enough) to keep their heads above the dark deep water waiting to swallow them.
Though they never talk about it, Light knows that L also knows that they're just a little (or perhaps a little more) afraid of what will happen to them if they ever let go.
It is not perfect. It is not bliss. But sometimes, only once or twice at all, Light wakes before L. Their hands are still together and L is still and silent and completely human, curled in on himself even in sleep. On those mornings Light can't help but stare at him, enraptured by the curl of his eyelashes and the thin slash of sunlight across his throat.
And he thinks that maybe, just maybe, this is happiness.