When Aang smiles, everyone feels better. It's almost as if there is no war to fight, no Fire Lord and his psychotic daughter to conquer, no prisoners (friends and family) to free. It's almost as if they are simply eight children and two very quirky animals, sharing a camping vacation in a deserted air temple. No one thinks about why they are there in the first place; no one thinks about why the temple is empty and silent.
When Aang smiles, everyone smiles back and Sokka feels inspired to say something, and Toph feels compelled to make fun of whatever he says, and Teo laughs, and Katara plays peacemaker, and Aang just keeps smiling.
Aangs knows this.
Aang also knows there is a war to fight, a Fire Lord and his psychotic daughter to conquer, prisoners (friends and family) to free. Aangs knows they are eight children and two animals huddling together in hiding at a deserted air temple. Aang knows they are there because they have failed too many times in the past and this is their last chance and they are the world's last hope; Aang knows the temple is empty and silent because he failed too many times in the past and this is his last chance and he is everyone's last hope.
When Aang smiles, he knows that The Duke and Haru are a little relieved because they are unsure about everything now and wonder if they'll survive to win or lose the war, and Zuko is slowly learning to relax around the others because even if they're not his friends yet, they are his allies and so deserve his trust, and Aang smiles at everyone, and everyone smiles back because when Aang (hope savior chance luck weapon protector Avatar) smiles, they feel much better and a little less afraid.
Aang knows all of this.
This is why he smiles.
If they are too busy smiling back and feeling better to notice how fragile the smile is, how quavery at the edges, how thin in the middle, or how it never quite reaches his eyes anymore because it is frozen by the bone-chilling numbness that is fear in his heart, he doesn't complain.
If they aren't, they don't mention it.