The light and hope in her eyes from that time have vanished. That was one of the first things he noticed from her when she first came here. Part of it is Mamoru's fault, but the main cause is most likely Nagawa Sano.
It seemed they entered a relationship shortly after he left, so Yuhi understood. Sumire was lonely; she didn't want him to go. But to the very last day, she never admitted such things. So she went ahead and randomly got a boyfriend. It didn't have to be Nagawa Sano; she just needed somebody who would help her forget.
The reason why he is having a hard time with Nagawa's case is this. He felt bad about it. Sumire never liked Nagawa Sano that way, and Nagawa must have noticed it. At least whatever feelings she had for him were not romantic, which caused friction in their relationship.
