Your Habour
There are two things that Sokka would die to protect. Three if you count the world but, as he figured that most people would die to save the world, that isn't really all that special.
He would die to save his village, if he thought it would help.
Some days he would die to save Aang, other days he thinks he'd just wait for a less annoying reincarnation. He might die in order to kill a large number of firebenders, but only if it was a super-massive amount and he'd really prefer to live through that. Once or twice he wished he would die in order to end the embarrassment of being a teenage boy, but that quickly passed. He was never quite sure if he would die for these things, and hoped it never came to the test.
He would die to protect Katara, if only she would let him. But she never would.
It's painful, he reflected (he does reflect sometimes, but it is rare, and usually only when he's standing guard alone staring at the wheeling stars overhead and listening to Katara or Aang cry in their sleep) to realise that your baby sister is in huge danger, and even more painful to realise that there is little you can do to help. She is more powerful than him, and would deem it her duty to protect him, and that thought hurts deep down right next to the thought that he should have been able to save mum.
"Take care of your sister." Dad had said, and left. "Take care of the village." The other men had said, before they sailed off to die. He just wished they'd told him how.
Sokka tried, he really did. He beat up Zuko, and banished Aang (okay so that turned out to be a mistake, but at least he tried) and tried his best to keep them fed and out of danger. He listened to her ramblings about random pretty boys, and tried to keep the creepy psychopaths away from her. He used science to protect his magical sister, and wondered why it nearly always failed.
Stupid magic. Stupid Avatar. Stupid Fire Nation. Stupid war. Stupid Katara, so ready to save the world.
He caught her when she flew too close to the sun, held her whilst she cried tears of frustrated hopes, swung his weapon at the phantom monsters chasing them and wished he could do so much more. He was meant to protect, and ever since this crazy journey started he had been only the protected. And whilst he knew that Katara would always love him, even if he was totally useless, he wanted more than anything to be the one she looked to in times of need.
He would figure out how to protect her, one day. And that would be the day when he would stand tall in front of her, and know that he was truly being her elder brother.
And all would be well.