Maybe they ran their course.
Maybe they were only allowed so much time together. People broke up for worse reasons all the time; people broke up for no reason all the time. They didn't exactly have a reason to break up, but they didn't have much of one to stay together either.
Except she loved him.
She loved him a lot, she loved him more than she thought it was possible to love another person. And maybe he loved her too, she didn't know, that was the problem. How was she supposed to love him when it felt like he didn't love her back?
And that wasn't really his fault, and she knew that, but she couldn't--handle it, she couldn't handle the distance that she always had to bridge, she didn't want it coming down to her all the time, that wasn't--okay. She loved him, but it wasn't just up to her.