Quote 1
"I'm having the strangest sensation," she whispered.
No—was it already too late?
Her eyes narrowed into the shape in his sketch and she came back to him, her lips parted expectantly. "Tell me I'm mad, but I swear I've been right here before…"
So it was too late. He looked up, shivering, and could feel the dark descending. (Prologue.35-37)
From this interaction, it's clear that this is a tipping point for Daniel. Every time Luce says this to him, it must put his guard up, because it's a signal that she might disappear at any moment. That really puts a damper on an otherwise uneventful evening, doesn't it?
Quote 2
"Um," she hedged, racking her brain for a sensible lie. Finding nothing. She racked her knuckles.
Daniel cupped his hand over hers. "I hate it when you do that." (4.90-91)
Although Daniel does his best to hide how much he knows about Luce, he sometimes lets his knowledge of her slip out without meaning to. It must be really hard for him to pretend that he doesn't know her, especially when he's known her for hundreds of years.
Quote 3
"I can't get it out of my head," she said, rolling over on her side to face him. She didn't feel steady enough to sit up yet. "This feeling that I know you. That I've known you for a while."[…]
"Haven't we been through this already?" His tone had changed, like he was trying to laugh her off…"I'm flattered that you feel like we have this connection, really. But you don't have to invent some forgotten history to get a guy to pay attention to you." (8.133-134)
At this point, Luce's nagging memories are coming to a head, and instead of telling her the truth, Daniel shuts her down. Again, we know why he's doing it, but that doesn't mean it's not frustrating as heck to watch them do this dance in circles all the time.