That can't really be helped. She was the only child in the village, unsurprisingly in that world, and was thus spoiled rotten. She was also early on given an important responsibility, so instead of simply feeling like a child, she also felt she had special authority. As if that wasn't enough, just look at Scar's, a pure gravekeeper's, behavior and it's evident gravekeepers's heads are a bit special on a genetic level.
But it's true I can't help but think something bad is going to happen to Ai when she's so heedlessly wandering around, not suspecting anybody, at least not enough.