The narrator says that there are three stages of love, each one orchestrated by chemicals in the brain. The first stage, lust, is brought on by testosterone and estrogen. The second, attraction, is fueled by dopamine and serotonin. Dopamine makes people feel happy together, while a drop in serotonin levels makes people obsessed with each other. The final stage, attachment, happens with the release of oxytocin and vasopressin. Both are released after sex and help people bond. Natasha knows that love is just chemicals, so she wonders why Daniel feels different.
When the novel discusses the science of emotion in this way, it makes it clear that emotion and rationality aren't at all at odds with each other—they inform and influence each other. This mirrors the novel's overarching message about passion and reason (that one should strive for a happy medium between the two) and shows that even on a molecular, scientific level, this still holds true.