His tears were like constellations: only spotted by a practiced few.
He wasn't so much overcome by anguish or grief; as a warrior, he had been taught not to dwell on your losses. He was thrust into the place of caretaker, with Pakku and his Dad excused for their emotional attachments, and Katara taking a much-needed break from her mothering position. His job was to help the others move along, become the leader.
But he couldn't help but feel…bare. Gran-Gran had been such a common part of his life that it was like looking in the mirror to find one eyebrow missing: you didn't notice until it was plainly obvious. He'd walk into the kitchen and expect her to be at the fire making stew, but find it was another cook. He'd sit at meetings and expect to see her at Pakku's side, but the chairs had been shifted and it was another waterbender in her place. After a day of fishing, he'd return to the house and call out names as a way of announcing his return, and only at the last minute remember not to say hers.
But as much as these moments bothered him, he didn't want them to stop.
It was this frustration that drove out the rare tears; he had to encourage everyone to move on, but at the same time…he didn't want them to.
Because if everyone moved on, wouldn't everyone forget?
But would it be better for them to forget, if all memories brought were sadness?
Why did all the people he loved have to leave him?
But he couldn't dwell on his losses. He had a family to pull together, a tribe to lead. He was a warrior. For his grandmother's sake, Sokka would soldier on.