Humans cannot survive without anybody by their side, even those people who wish to be alone and behave coldly. They still need warmth and daily interactions with people; otherwise, they would go insane. It's not surprising that Sumire broke down after a week.
She may have lasted longer in the past. But ever since she arrived in Tokyo, he has made sure that she experiences nothing but warmth and happiness being surrounded by people.
"So, Ru he, stayed by my side every night. At first, he simply stayed and kept guard by the door. Then I persuaded him to at least sit on the couch. Eventually, he couldn't ignore my violent attacks at night and slept by my side. Oh, but nothing happened. He didn't even want to hold my hand at first."
"You don't have to worry. I know Mamoru isn't the type of person who would take advantage of you."
