Johnny and Sarah had loved each other very much.
But things change.
For Sarah, it had been years. She'd moved on, with Walt, and had started a family. Of course she cared for Johnny deeply, that would never go away, but the capital R Romance had expired, like Johnny's driver's license, and this one was so much harder to renew. And that was hard to accept.
For Johnny, it had been hours. He remembered all these little details that she'd surely forgotten. Like it was yesterday. And it was, to him! But… time had passed. It had really, actually passed. She was a different person now. And that was harder to accept.
There's this confusing empty feeling. Neither of them want what has happened. They want either to run it back, not have the accident, and live out their perfect little life they had planned or, they want Johnny to have gone away peacefully and let Sarah have her life that she'd built instead run uninterrupted. This halfway may have seemed like a compromise, but it's harder on them both.
That hurts, for Johnny to know that she'd be better off if he'd died. But he can feel the weight of it on her every time they hug, brush against each other, or he touches something she's touched.
And that hurts, for Sarah to see him go away for a bit every time. She doesn't quite know where he goes, but all it does is remind her that things are different now.
He's not the same person he was 6 years ago. And she misses the Johnny that was.