Natasha is both skeptical and intrigued when Daniel tells her about the study. Daniel asks what marriage is for if love doesn't exist, and Natasha says that it's about mutual self-interest and the benefit of the next generation. She thinks that Daniel keeps getting cuter. He asks what the ingredients for love are, and Natasha suggest mutual self-interest and socioeconomic compatibility. Daniel offers his own ingredients: friendship, moral compatibility, intimacy, physical attraction, and the mysterious X factor.
Though Natasha's ingredients for love are unsurprisingly rational, what she says also offers some insight into what she likely has observed about her parents' relationship. This suggests that her parents' relationship doesn't include actual love or attraction at this point. Given how she used to idolize her dad, a loveless marriage could be especially hard for Natasha.
Daniel asks if he can have the day to make Natasha fall in love with him. She says he can have an hour until her appointment, and then she has to go home. She ignores him when he asks what the appointment is, and finally says that she's not going to tell him. Natasha almost admires his tenacity.
Natasha's unwillingness to tell Daniel about her appointment indicates that it's actually immigration and deportation that's in opposition to their budding romance, not reason.