[It wouldn't be wrong to say he's taken aback by the outburst, though perhaps less than someone else might be -- he knows Korra, after all, and she's the type to say her thoughts directly. But the idea that he wouldn't understand? Wouldn't get how it feels, to suddenly have everything you've ever thought you were good for pulled out from under you, to have to face the fact that you're not needed anymore, that everything you ever thought was the good you'd done in your life, you wouldn't be needed for it?
Don't talk to him like he doesn't get that. Protecting a friend and being the bridge between worlds might be two different things, on a very different level... but fundamentally it's the same.]
You go on! You-- you think there's anyone who'd always be the same and never change? You think I don't know what it's like to go through hell, find yourself on the other side and realize all you were ever needed for isn't there anymore? [He pauses, breathing hard while he tries to catch the thread he was going with, here -- talking about this, even indirectly like this, still throws him off the loop.]
-- You can't go back. The only thing you can do is go forward. Even if you don't know how. And even if they tell you you're not needed -- if you know they're wrong, that's what you hold on to.