Imagine you're one of these characters in a book. Just to make it easy, let's say you're the girl. You've met the perfect guy. All sorts of crazy plot things have happened, and now it's the end of the story, and you guys are a couple. And the best part? You're not just any couple. You're the perfect couple. No one will ever keep you apart. You are like Prince Charming and Snow White. Life is good. And it will be always.
Step back away from yourself and your perfect relationship for just a second. What are the odds that this guy is the one? Like he's the one you're destined to be with until you are old and gray?
You think the odds are pretty good? Okay, fine. I dunno. Maybe. Maybe you are "that couple." The one that sticks together through it all. High school sweethearts. Whatever. I wish you the best of luck.
Here's the other thing. You know how your guy has been kind of perfect through the story. Sure, maybe there is one slip up. One little thing he does to show his anger, his dark side, the demons he has inside. It shows us he's imperfect. This is good.
Let's take a closer look at this. See, an author wrote your story. That author designed both you and your perfect match. She (or he) came up with the words that came out of your mouths, planned out the things you did. And she (or he) made sure that, for the most part, those things were likeable. And relate-able. And she (or he, whatever) made sure your guy didn't do too many things that might make him appear to be a jerk. Because if she (he) did, then readers wold complain.
For the record, in no way am I saying that guys (or girls) are jerks. I'm saying that people are human. And as such, they have up days and down days and sometimes they say or do things they regret...for no real reason except that they're having a bad day. But as an author, trying to capture this type of situation for a character becomes difficult. And is thus sometimes avoided.
Still, we don't want any Mary Jane's, and that's why often times you'll see these totally planned, individual scenes to show our characters' imperfections. Something to bring them into the realm of "human." But still they do not cross the line. They are sort of like demigods compared to us. We flaw all the time. Their flaws are numbered like the natural satellites surrounding Earth.
There is only one moon going around Earth
It's true that desperate situations make people grow closer, and books are often filled with just these kind of desperate situations. These situations bind characters in ways no one else can understand. Yet, after I finish a book, I can't help but playing the "then what" game. So write your romantic stories. Seal the ending with a kiss. But then what?