Haruki hears about Kazusa's moving from her mother, who she hadn't been on good terms with.  He feels betrayed and angrily confronts her.  But his words force open the door to Kazusa's heart, which she had desperately tried to keep closed so that the three of them could stay together.
I don't see how you can screw that up.
Ich versteh nicht, 
wie man das vermasseln kann.
No veo cómo puedas echarlo a perder.
¿Cómo puedes ser malo en eso?
Je ne vois pas comment 
on peut se tromper.
Não sei como dá para errar
 uma coisa dessas.