I don't think its like that per se.
This is a story of young love with a bittersweet ending. We don't really get THAT much of a backstory on these two, we only get snippets of how the relationship is, and to me it was very pure. You could argue childlike. They were happy with how things were. Any problems they might have, they probably thought could be solved with more time. Remember, these are middle school kids, so must likely that have a very pure/childlike view on relationships.
But Tokyo happened. While it doesn't say it explicitly that its that far, the way the story implies is such a big deal he's going away to Tokyo, its going to be a ways away from their hometown. All that time they thought they had was gone, and there was a very possible end date. I think before he was interrupted, he was going to break up with her. The way he was talking and the face he made when she brought up "next time" means that he's been putting off this talk for a while.
Then things got a little real. The problems in their relationships had to be addressed. And had to be addressed NOW.
We then get 10 years later. Their lives went on in different directions. They seem to be happy, but we get the feeling they do have some regrets on how it ended. On what could have been.