Some children (arrogant, and powerful, and broken) come to Iruka, already understanding what it means to a shinobi.
Most do not.
Iruka's eyes are clear. He knows that a good shinobi can not be a good man. Careful, Iruka says, be careful--and he teaches his children that the world is dangerous, and that they will grow to be yet more dangerous. And because they are young, and Iruka's contradictions are artfully managed, his students will grow strong and fearless.
They leave him, their dreams polished, refined, and intact. Understanding--true understanding--will come later.
It will be far too late, for Iruka will have already taught them belief.