Harry actually stopped breathing at her words for a full twelve seconds ... his body tense as if under an oppressive curse. And he was so close that Hermione could see his pulse hammering relentlessly in his neck, his throat full from dry shock. He just didn't know of any way to respond to Hermione's oddly-timed declaration. She watched him a moment, waiting for a response, then understanding immediately that he was utterly incapable of anything of the sort.