He gazed at the black-haired girl, trying hard to maintain the sincere and earnest look from earlier: "The matter your father brought up just now, I... I also didn't expect it, it was a bit abrupt, so instinctively I felt it was a heavy responsibility, not knowing if I could bear it, I didn't really intend to shirk it..."
So that's it...
Was it because it was too sudden at the time, so he said things like "I can't" and "I can't do it", now realizing he said the wrong thing, worried she'd misunderstand him, so he's eager to explain?
Miss Saito's eyes were bright, she thought this was probably the case, but if it was, then she might feel a bit embarrassed instead. Although he was the one who mistook her first, her mischievous pleasure seemed to be part of the reason... Such words should be said to her sister, it felt strange saying it to herself.
