Ryuk quotes from Death Note

You've taken quite a liking to it. (Ep. 1, first words to Light)

Look at how many people you've killed in only five days. Most are reluctant to write this much. (Ep. 1)

Let's just say this: you will feel the fear and pain known only to humans who've used the notebook. And when it's your time to die, it will fall on me to write your name in my death note. Be warned any human who's used a death note can neither go to heaven nor hell for eternity... That's all. (Ep. 1)

I didn't choose you. Don't you see this is all just an accident? You actually thought you were chosen because you're so smart or something? Don't be so vain. It just happen to fall around here and you just happen to pick it up, and that's all there is to it. (Ep. 1)

You're asking me why? I did it 'cause I was bored. (Ep. 1)

Humans are... So much fun. (Ep. 2)

You have lost, Light. Didn't I say in the beginning... when you die, the one who'll write your name down in a notebook will be me? That is... the deal between the Shinigami... and the first human to get their hands on the note in the human world. Once you enter the prison, I don't know when you'll die. It's annoying to wait... Your life is already over. You'll die here. Well, it was good while it lasted... We killed some boredom, didn’t we? We did some various and interesting things... (Ep. 37)

Well, Light, it looks like you've lost. Remember how in the beginning, when we first met, I told you that I'd be the one writing your name in my notebook? That is part of the agreement, between the Shinigami who brings a notebook to the human world... and the first human who picks it up. If they put you in prison, who knows when you'd die? And I don't wanna lie around waiting. So, it's all over. You'll die here. It was good while it lasted. We eased each other's boredom for quite a while. Well, Light, it's been interesting. (Ep. 37, as Light dies, the last words in the anime)

Ryuk

Why even bother? What are you trying to achieve by passing judgement on them? I mean, why do you care?

Light: Because I've been bored too, Ryuk. (Ep. 1)