Reading about Aang wordlessly crying from horror when Zuko explains to him how he got his scar and why he spent so long trying to capture him is cute and all, but we are talking about the same kid that went into the avatar state because some morons had the audacity to muzzle Appa, so please consider this:

His face falls, something dangerously close to a scowl forming on his face and, for a second, his tattoos and eyes shine, the light almost blinding, but as soon as it was there its gone, so quickly that if you blinked you would have missed it, and now he just looks tired and sad. 

He thanks Zuko for trusting him enough to tell him, his voice soft and quiet and maybe shaking a little bit, and then he just hugs him, as tightly as he can.