Sokka had been so busy making Appa's armor and working out last-minute kinks in the invasion plan that he barely noticed Aang wasn't himself. When Katara confronted him, though, he saw it immediately.
Aang's eyes were sunken and wild in an overly-pale face, shaded by dark bags. His stances were getting worse and his shots going wild with his manic energy.
Sokka wouldn't express worry, of course – that wouldn't be manly. But he didn't protest too much when Katara insisted they help him relax.
If Aang couldn't calm down, the invasion would fall apart – but, more importantly, so would Aang.