Here, falling for someone is treated almost like a science experiment gone wild. The speaker sees the person they desire and feels their whole body react: cells "rising up," nerves sizzling, heat gathering in the chest. The title, "biological reaction," fits the way the speaker describes love as something physical and instinctive, a "primal need" that arrives before any thought can step in.
The first stanza is one long, breathless sentence that builds and builds, mirroring the rush of feeling, until it breaks off mid-thought at "bursting out as." Then comes the surprise. The release is not a grand gesture but a simple moment: the beloved smiles, as they always do. The speaker's body "slows with a shuddering jolt," and all they can taste is sweetness, repeated in "you, you, you." The mood is giddy, intense, and a little overwhelmed, a feeling so big it leaves the speaker nearly speechless.