Aang learned to let go of the past, just as he promised Katara on the day the storm ended, and he kept that word in his mind and in his heart for ever since. He looked forward to the future, enjoying the sights of the present, resolving himself for the battle against the Fire Lord to end the war. Nostalgia would not occupy his thoughts during those times, especially when everything else started to fall apart when Ba Sing Se fell.
But then, when Aang found himself alone in his thoughts and his friends occupied with their own devices, he would allow himself to close his eyes, and see the Western Air Temple before the war in his mind. The buildings whole, the nuns milling about minding their monastic chores, and visitors from the Fire Nation would wander along the now empty halls. He last saw Kuzon here, a hundred years ago, excitedly telling him that his dream of being part of the army, following his older brother's footsteps, would finally come true. He was fifteen that time, around Sokka's age... and was told that he would train formally the next year.
Aang could not allow himself to think if Kuzon personally saw how the nuns and their charges died. The Western Air Temple would be the first sanctuary to be attacked for being in such a close proximity. It was too cruel to contemplate such a thing, for both of their sakes. And so he imagined that Kuzon got sick, or stubbed his toe that hindered his walking, or ran off and became a clown in the circus, anything to assuage himself that his best friend was not involved with Sozin's insanity against the other nations. And he drew a small comfort from that. Unless told otherwise, his memory of his Fire Nation friend was safe.
It's funny. Aang thought, Zuko sort of reminded me of him. True his firebending teacher does not have Kuzon's sense of humor, a trait that made them close, but his tendencies of bringing all sorts of trouble when they trekked the Sun Warrior temple together was eerily similar to their scrapes a century ago. Aang smiled in spite of himself, Kuzon would definitely like the former Fire Prince.
In his mind's eye, he saw Kuzon waving goodbye. He promised to tell him more stories from his training the next time Aang would visit, neither of them realizing that they'll never see each other again, with the world forever changed.
"Aang!" Sokka's voice rang out, breaking his reverie. "Dinner's ready!" the Avatar opened his eyes, and the vision of the past disappeared. He took a deep breath to steady himself, and as soon as the Water Tribe youth stooped over from the pillar to look at him, Aang was already smiling.
Sokka was disgruntled. He disliked having to be kept apart from a good meal, but sensed a hint of sadness from his friend. "Something wrong?" he asked, not fooled by Aang's grinning expression.
Aang easily shrugged his shoulders, looking away from Sokka's keen observation. "It's nothing." He assured him firmly. Until the past comes running back to him, in which it was all it will ever be.