It's simple, really: Cullen isn't who he wants to be, so he asserts, loudly, that he is -- and the lie's enough to make him feel ill, unconscious though it may be.

And it's mildly infuriating (for reasons he can't quite articulate) that Alistair isn't having it.

I don't know how you can stand to be around me, he doesn't say. His shoulders are up around his ears. Why don't you get angry?