At a acertain point, he senses that she's not really there. That's the moment when they decide to separate. And from then on, she wants to win him back. It's precisely at that moment that she falls in love with him.
Had a hunch that it might be his best ever. :) And it is, indeed, at least I think, his best ever. 無論是quirky卻又無比美好的台詞, 到無可挑剔的art design和soundtrack, 都無法忍住看完即二刷的衝動.