What Happens to Characters After the Story?

In The Fault in Our Stars, the character Van Houten refuses to discuss what happens to his characters after a book ends. 

I listened to the audio commentary for The Fault in Our Stars movie. It's the first time I've heard an author speak on one of these! John Green says that he's the same way. He doesn't want to think about what happens to his characters after the end of the book. 

I can't think that way. All of my stories interlink, so characters often reappear in other stories. And I've written many short stories that go into events before and after the main story. It's pretty fun to do. Even if I haven't written a story yet, I may throw in references to it. So I often have to know what happens to many of the characters. 

That being said... there are some pretty big things I have no clue about. For example, I have no idea who my single characters end up with, if anyone (unless it's in the actual book). I'm working on a short story right now that takes place many years after the end of the main story. And it just occurred to me, I have to figure out if the two main characters are seeing anyone. Or married! I have no clue! I may just cheat and only vaguely reference it... 

While I'm here, I have a couple other comments on TFIOS. I once watched the movie with someone who I had thought was a friend. Isaac's girlfriend leaves him before he's going to have a nasty surgery. And the ex-friend was so matter-of-fact when she said, "well maybe she really just can't handle it." I was shocked. But after the way she acted, I can't say I'm surprised. 

Real love doesn't mean abandoning someone when they're going through a bad time. 

Anyway, I just wrote a post about Westside Story - a movie I'm not particularly familiar with. I've only seen it a couple times. After I watched TFIOS, I looked up what other movies the actors were in. To my surprise, Ansel Elgort (Augustus) is in the remake of Westside Story. I'd completely forgotten that there was a remake. I looked up a couple clips to his songs. Dang, he can sing! I guess I'll have to rent that movie now.