There's something that bothers Yagami Light.
It's the way he keeps thinking of the time Ryuzaki announced that he was his first friend.
It's not only that (why would he keep remembering it in the first place?), but also these amorphous feelings attached to that memory.
There's one feeling that he just has no idea about. It's exactly like a dream that's been immediately forgotten—the color of it is there, but the meaning is forever lost. This feeling bothers him often, because sometimes he feels it about Ryuzaki in general, when they're discussing Kira. It's a red haze behind his eyes, but he doesn't understand it, and so it bothers him.
The other feeling actually becomes clearer as he probes it, but the closer he gets to unraveling it, the more he desperately wants to stop. That feeling is dangerous. He knows he respects Ryuzaki, respects L. He knows he admires him, for his quick deductions that seem to come so slowly. But on some level he already knows what this feeling holds—knows that if he pried it open just a bit more, he'd realize that he was actually infatuated with Ryuzaki—so he never lets himself get that close to figuring out what it is.
Overall, it bothers him. Just as Ryuzaki bothers him. With his stare and his relentless accusations. But as the handcuff snaps around his wrist, he can't help but feel he's accomplished something wonderful.