Disclaimer: I don't own Death Note or any of its characters and trademarks.
Ryuk laughs at Light. That human is such a brilliant source of amusement. His constant battles with detectives that wish to bring him to an end ensure that there is never a dull moment for the observing Shinigami. He chuckles at Matsuda's blind faith in Light, is vaguely amused by the rose coloured glasses Light's father views his son through and almost impressed with L's lingering suspicion. Light works among them every single day with the intent of capturing Kira. This amuses Ryuk further because he knows that Light is more Kira than he ever was Light Yagami.
The young man puts up a front during those long hours, says the things he knows the task force members want to hear. He leads them all by their hands and they're just too amazed by the things he says to object. L is the black sheep, the one that refused to be led, the one that thinks he knows the truth. The others believe that L is being stubborn and far too persistent, that he just isn't willing to admit that he made a mistake in accusing Light. Ryuk laughs at the irony, thinking with a hint of smugness that L is the only one not making a mistake. Well, he couldn't quite say that. Getting involved with the Death Note in any way, no matter how unwitting or indirect is a mistake, even for the "all powerful" Kira.
Ryuk is content to watch as Light's god complex swells, as his desire to purify the world clouds his sense of consequence. He thinks he is elevating himself; Ryuk knows that he is actually digging himself an early grave. Any sane human would have discarded the Death Note as soon as they discovered how real its power was. Ryuk considers himself lucky to have his notebook found by someone like Light, someone so egomaniacal and so disgusted by his fellow humans that he would start such a crusade.
As godlike as Light thinks he is, he is actually as fragile as any other human and using the Death Note makes him even moreso. Ryuk wonders if Light even cares to recall when Ryuk told him so many months ago that he would be the one to write Light's name in the notebook. That time isn't now, isn't even close to now, but that doesn't stop Ryuk from predicting how Light will react as his final seconds tick away.
A Shinigami is a god. Any human claiming to be one is nothing more than a glass replica that can--and will--shatter at any time.