Aang's face was filled with pain at what he surely saw as yet another failure. Sokka couldn't deny that the Fire Nation knowing about the invasion was a blow – but there might still be hope.
When his father suggested Aang go after the Firelord anyway, Sokka watched the spark of determination flare in Aang's eyes. Katara shook her head in disapproval of the plan – but it was Aang's decision to make.
"Whatever you decide," Sokka told him, "I'm with you."
He watched Aang's face tighten in resolve, and knew his answer. The Avatar stood, and spoke.
"I've got to try."