Kazuha was your friend.

He had a crush on you since highschool, and now the both of you are in college.

He was shocked. Your best friend had confessed to him, but he loved you, not your friend.

He rejects your friend, and the news got up to you, your best friend a crying mess.

He was even more shocked when you were mad at him, comforting your best friend. He loved you, not your friend. That was the reason why. He didn't know you'd hate him. He feels guilt and sadness.