She doesn't understand Stephanie.
They're on patrol, or at least they're supposed to be. But Stephanie has paused on a rooftop. She's looking up at the sky and even though her face isn't visible beneath the Spoiler mask, Cassandra knows she is smiling.
"Look at the moon! Isn't it beautiful from up here?"
Cass looks. It's the moon, the same as it looks every night that it's full. She can't see anything special about it.
But then she can't see anything special about most things that Stephanie seems to love, particularly anything they did while on patrol. Yes, there is an endorphin rush when swinging on a jumpline, but she's never made the sort of excited gasps Spoiler sometimes does. She doesn't smile when fighting or feel a thrill (or even especially triumphant) when she wins.
Being Batgirl is something Cass takes very seriously. She works hard at it and believes in the mission. But she's never found it fun.
Stephanie is different. She often acts as though it's the best time she's ever had. Cass has tried to understand, even asked her to explain, but Stephanie's behavior still makes no sense to her.
She's too alert for anyone to sneak up on, but she still jumps when Spoiler drapes an arm casually around her shoulder and points at the sky with the other. "Look, a falling star. Make a wish."
She thinks that whatever Stephanie sees, it must be the same sort of thing Barbara sometimes talks about: maintaining a civilian identity and having a life outside of the mask, being a girl instead of a weapon. When Barbara talked about it, she'd dismissed it as unimportant, something that didn't really apply to her, even if the first Batgirl thought otherwise. They shared a secret identity, but they weren't the same, so while she didn't understand, it didn't matter.
But here on this rooftop with Spoiler, she wants to understand after all. She tilts her head a little so it rests against Stephanie's, and makes a wish.