[He can't help it -- he laughs, even if he has cause to immediately regret it.]

I'm a doctor, Mr. Stark, not a superhero.

[Just a little private joke for himself, there.]

If I could do it myself, I would, but I need someone with steady hands. I'm going to talk you through every step. Promise I won't even cry.

[He's joking. He really is joking.]