Daniel thinks that he's never noticed anyone the way he's noticing Natasha. Her hair looks like a halo, and her eyes are beautiful. He studies her face, thinking it's a good thing that she's too sad to notice how shallow and horny he is. When Natasha looks up from her headphones and meets his eyes, he feels a sense of déjà vu, but like he's experiencing something that will happen in the future. He can see himself and Natasha at every point in their future lives.
Natasha's emotions about her headphones work to isolate her, while Daniel's attraction to Natasha actually makes him feel deeply connected to her. This begins to illustrate the ways in which different emotions influence if and how people feel connected to each other. This passage also specifically shows that sadness is isolating.