The real reason
Sano quickly paused when he realized what he just said. Even though he didn't say it out loud, it was a first for him to have thoughts like this. He recalled the exchange he had with the girl not too long ago and exhaled deeply. He hates to admit it, but she is indeed right. Both of them have changed now. The reason why they can't pull away from each other is because they had a bad breakup.
Sumire no doubt has feelings for Terashima. But it is hard for her to move forward since she has lingering regrets. Sumire recognized this earlier on; she realized it before they first met again after so long. But there was a part of her who held bitter feelings, a strong and powerful emotion. She acted on the latter instead and chose the more difficult route.
It seems whatever happened in the institute the last two months led to her changing her mind. He doesn't mind that outcome, but Sano clenched his fist.