Aang really is such a precious spirit.
While learning to control his Avatar State,
meaning the point where his energy is most powerful,
he learns from his teacher the 7 points of mastering his chakara.
His teacher tells him of the seven "gates" in the body, and how to open them so his chakara can flow, as well as what constricts or binds them.
When Aang learns of the Air chakara, his teacher informs him the region is the heart, and the gate is opened by love, but blocked by grief.
Aang sees a vision of all his tribe, the air benders, fade away into the clouds, leaving him alone as the last of the air benders.
But, before the grief is overpowering, the clouds gather and Aang see's the clear image of his deara friend Katara smiling back at him.
What is blocked by grief, is opened by love.
Aang then cries soft tears and wipes them away with the most precious smile I have ever seen.
Since mastering the Avatar State demands one must release their earthly attatchments to embrace the cosmic energies, Aang must surrender his love of Katara, to be free of all earthly bonds...
Of course he refuses, but in the end.. you'll just have to watch it. ^^
For those who watch Avatar, you'll recognize the necklace arounds Aang's neck as Katara's grandmothers necklace. Her proudest posession. Which makes this image a simple speculation of fanart since he does not actually posess her necklace, and is in fact in his Avatar State (thus the glowing arrow on his forehead lol)
I had to add the tears though, the image this is based off of was just to precious to leave them out..