aang and kyoshi were the only recent avatars to come into power during wars.
kyoshi with chin the conquerer and the earth nation war, and aang with the fire nation. they had to grow up and take it seriously. but kyoshi didnt grow up in a culture that honored every living thing and believed violence was never justified, to the point that they were vegetarian pacifists without a standing army.
kyoshi’s advice to aang was practical. people will die, you have to act, you have to be the justice in the world. aang then realizes he needs spiritual advice, not practical advice.
yangchen, the other air nomad avatar, tells aang that yes, their people believe in spiritual enlightenment as the end goal, and that it’s extremely important. but that the avatar cant do that, so, give it up. its just not in the cards for you.
but yangchen had other nomads around her- she could in a sense, martyr herself to save the world and protect her people as a warrior.
aang doesn’t have that option. he can’t be a martyr- he has to live, both physically and spiritually, to ensure his culture and his people live on- to ensure the avatar itself lives on. he has nothing and no one to fall back on.
yeah he was already extremely mature for his age and took it incredibly seriously as a child- he was a mild-mannered, gentle spirit, who, as we see through other avatars but most notably his successor, korra, and his son’s family, was already extremely adept and competent at manifesting the spiritual side of being the avatar. aang’s spirituality is incredibly important to his character in ways we dont even truly realize until after he’s gone.
and so, to preserve himself and his people, he does what humanity needs him to do, what humans have always had to do since the dawn of time when the need is greatest. he evolves.