The repetition is a little annoying, but the ideas presented are actually very deep philosophical ones.

I got the concept of the reason the girls were committing suicide. There are similar versions of this story everywhere, but one I remember the best is from the Chuang Tzu.

A frog lives in a well, and is convinced that the "world" which consists only of this well is the greatest place. It's everything, all encompassing, and there's a tiny patch of sky visible. One day, a turtle visits the frog. The turtle has come from the sea, and tells the frog how truly vast the real "world" is. The frog doesn't believe the turtle, and remains deadset that the well is the entirety of the world.

Kirie was forcing these girls to "fly" and see the true vastness of the world. As Touko pointed out, when people see the real vastness, they are often completely overwhelmed, and can't comprehend it at all. So they "fly." Kirie filled them with the delusion that they really wanted to do this, and could fly.

Kirie herself was much like the frog. She could only remember seeing the patch of sky and the part of the city from her hospital bed. Some man came and offered her a second soul that could fly around in the city, or at least the top of the building her parents used to own. So she believed that she was "flying." But as Touko explained to her, she was really only floating, as she was still very much attached to the world, and just bringing others down with her (though the girls actually did "fly" when they died). Her suggestion was that Kirie actually go and "fly" instead of merely hovering about like a ghost.

When Shiki instantly killed Kirie's second, ghost self, Kirie returned to being that frog in the well. But a part of her remembered the delusion of "flight" so she killed herself in the end.

The reason the illusions didn't work on Shiki the second time is she claimed to have no desire for that sort of thing. It's understandable. With the Eyes of Death Perception, Shiki already knows quite a bit about how vast the world truly is. She has also obviously chosen to stay right where she is, with (or at least as near as she can be) Mikiya.