Natasha admits to the reader that she didn't tell the truth about what she'd do with a time machine. She'd ensure the best day of Samuel's life doesn't happen to save her future. When Natasha is finished explaining the science of time travel to Daniel, he looks like he's in love. She's surprised he hadn't heard of the complications of time travel, but reasons that she wrongly assumed that he's nerdy because he's Asian. She tells the reader that she's not an appropriate partner for anyone, especially since she doesn't like temporary and non-provable things like love.
It's telling that Natasha links her dad's "best day" directly to their deportation. This suggests that she believes fully in a system of cause and effect in the universe, though she’s unwilling to refer to it as fate or destiny. Meanwhile, her misconception about Daniel shows that everyone is capable of making assumptions about people different from themselves.
Natasha admits to the reader that she's not sure she's capable of love. When she was with Rob, she never felt like the songs said she was supposed to. Very seriously, Natasha tells Daniel to not fall in love with her. He sputters. When Natasha explains she doesn't believe in love, Daniel insists that it exists whether one believes in it or not. He says that poetry, love songs, and marriage prove that it exists. Natasha counters that it's impossible to test love in empirical ways, and that a misguided belief in love is how people avoid admitting that life has no meaning.
Natasha's reasoning is decidedly unscientific; no scientist would take a sample size of one seriously. This again begins to show that Natasha probably has other reasons for not wanting to believe in love. Questionable scientific practices aside, her experience with Rob is a convenient way to justify what she wants to believe is true.
Natasha declares that she's fine not believing in fate or destiny, and Daniel laughs. He seems almost happy about her nihilism and admits he finds it interesting. Natasha closes her eyes and listens to the Pearl Jam song playing in the shop. When she opens her eyes, Daniel says he knows how to get her to fall in love with him scientifically.
It's also worth noting that within the world of the novel, it doesn't matter if someone believes in destiny or not; it still works on them. This shows that Natasha can absolutely believe whatever she wants, but it doesn't change the truth of the world she lives in.