–The scene Harumin sees in the final chapter is one of Punpun surrounded by friends. Looking at that one scene on its own, it’d seem like a really happy one.
Asano: Exactly. But in reality, Punpun never had anything go his way in his entire life. Not once.
–Right.
Asano: After Aiko died, what he really wanted to do was just live a lonely life mourning for her, but in the end he gets caught by Satchan and it all gets sort of muddled. Punpun is broken at that point. When he talks with Aiko in his dream (chapter 145), he says something along the lines of wishing he could just disappear from everyone’s memories, but even that wish isn’t granted.
–The phrase “Good night, Punpun” recurs multiple times throughout the manga, but in the end, when Punpun himself finally chooses to mutter “Good night”, it gets overturned by Satchan. You could say it’s the story of someone who chooses eternal sleep by his own will, but gets woken up anyway.
Asano: That’s true. Also, I might have made it a bit less than obvious, but Satchan is making Punpun into a manga. She’s digging up Punpun’s life, which he wants everyone to forget, and turning it into something permanent — manga.
–Considering Punpun’s personality, that’d be a living hell for him.
Asano: Right to the very end, I wasn’t sure how to go about doing the last chapter. Among the possibilities I’d considered, I’d thought up an ending in which Punpun dies.
–How?
Asano: Satchan’s child falls off a train station platform, Punpun goes down to save him, dies instead. It’s a very clean way to end. But I wasn’t sure if I wanted to end on such a clean note.
–What do you mean, “clean”?
Asano: It’s too clear-cut an ending for the story. It wraps it all up a little too well. Living is harder than dying, see, so I thought this was the most painful, worst possible ending for Punpun, and that’s why in the end I went with this final chapter.
Here is the link if you are interested in reading the author's interview:
https://mangabrog.wordpress.com/2014/07/06/inio-asano-interview-reality-is-tough-so-read-this-manga-about-cute-girls-and-feel-better/