He wanted his beliefs to be reaffirmed.

After speaking to multiple past Avatars who also take a stance opposite to Aang’s, Aang says:

All these past Avatars, they keep telling me I’m gonna have to do it. They don’t get it. Maybe an air-nomad Avatar will understand.

He, arguably showing signs he thinks their culture is more enlightened and morally superior to the others, expects the previous airbender Avatar to agree with him, and that to be enough for him to stick to his guns and ignore the advice of other Avatars, his friends, and the wider world (aside from the fire nation lol)

It’s only when an airbender Avatar very bluntly says he needs to do it that he finally capitulates and accepts the advice everyone gave.

Aang is wise, but he still has ego, biases, and other human failings.