That's 'cause 'e can't see the wood for the trees. Since when does curious need a reason?

[She's speaking of herself here, more than Cain; but she's recognised the signs. Curiosity, abstract and catholic, has been Iris' lifeblood all her life and she's familiar with all its flavours - the shrewd, investigative Cain she watched speaking to Hannibal is a new one on her and perhaps, she suspects, on him too.
Not that it hasn't always been in him; but she rather thinks it's always been turned to whatever end he had in view. Possibly it's a surprise to him to find that out of all the tools turned to his trade, this one's still ready to his hand.]

...It's one of the few things I don't like so much 'ere. No real scope for investigating. Finding out 'ow people work, though. That's never not fun.

[She laughs too, and runs her fingernails from the base of his ribcage around to his back.]

I like a nice bit of liver meself. I'm rather glad I weren't in the infirmary when 'e did 'is food run.