There are no butterflies or fireworks when he kisses her. It's nice, don't get him wrong, but it's nothing… special. It's nothing spectacular, or exciting.
After all they've been through, he knows enough to tell that this isn't fate.
But he stays, because the only other thing he can do is imagine the who is special, with her blonde hair and dry sarcastic comments, the one who stayed because she wasn't as selfish as they were, who hadn't put herself first. He thinks of her and wishes he hadn't made the biggest mistake of his life all those years ago.