"It's a new side to you. It might not be polite conversation, but I also know you won't say something you don't want to say."

Light is worried that L will find himself bullied in this scenario. While the man behind Kira is accustomed to being able to see the mood, L isn't someone who makes it very easy if Light manages it at all. Thinking faster than most normally gives Light an edge, but L's able to keep up. It's both annoying and exhilarating, though there isn't a lot of deception in the surprise so far. Light really did assume this a side of L that doesn't exist.

And like so many of their interactions, L lays a puzzle in front of him. Some might think that the vague replies mean that the detective would prefer the answer remain a secret, but Light knows it isn't true. L likes to watch Light work, enjoys the process behind the puzzle. L isn't dropping clues so that it won't be discovered; he's dropping clues so that Light can glue together the pieces.

There's a moment before all the edges align and the answer becomes clear. A surprise indeed, each seeming to top the last. At least, if Light's right.

"Medication."

The pills. Of course he remembers them.

"You've been taking medicine to suppress it?"

Is that also why L looks so young? The side-effects of suppressing the urge could be that supression of the body.