Zombie YA somehow always swings between gore and butterflies in the stomach, and the balance hinges on where the dread is sourced. Early 'Rot & Ruin' books come to mind—the horror isn't just the shambling dead, it's the moral decay of the survivors, the choices you make to stay human. Romance blooms in those cracks, a quiet defiance against the grimness.
Contrast that with something like 'The Forest of Hands and Teeth', where the romance is almost a secondary infection, a desperate cling to hope in a claustrophobic, cult-controlled world. The romantic tension gets its charge from the ever-present physical threat; a stolen kiss feels like a rebellion because the fence could break any minute. The imbalance happens when one element becomes wallpaper. If the zombies are just a backdrop for love triangle drama, it loses teeth. But when the relationship's survival is tied to outsmarting the outbreak, the stakes feel real for a teen audience. I dropped a series once because the lead spent more time worrying about her prom dress than the horde at the gate.