I am here to call bullshit on that.
So here's how I go about creating stable but dynamic relationships. It's a two-fold process. The first part is with the character creation. The second is with the plot structure.
The Kowal Relationship Theory
My mother-in-law, Patricia Kowal, came up with this as a way to describe who you should look for while dating. The theory is that relationships exist along multiple axis and the more closely aligned you are on these axis, the more you'll get along.
- Mind - Both people have similar levels of intelligence.
- Money - Both people have similar attitudes about money. They don't both have to have the same amount. This is about what money is for and how it's handled.
- Morals - Similar moral compasses of right and wrong.
- Manners - Similar senses of what is polite. So it's possible to have the same manners and wildly different morals. (These are the people that you meet and they are charming in real life, but assholes on the internet. You can't see their manners online.)
- Monogamy - Similar attitudes about the relationship. You know that guy that thinks you are BFFs and you think you're just colleagues?
- Marx Brothers - You both find the same things funny.
Now, any that you are out of alignment on is going to be where your points of conflict arise. For instance, Rob and I are closely aligned on all of these BUT on the money spectrum, we agree on what money is for, but are out of alignment on how it should be handled. I am the grasshopper. He is the ant. This is where most of our conflicts occur.
Jane and Vincent are in alignment on everything except some aspects of manners. He is unequipped to deal with her family and that's where their points of conflict tend to arise. Also, the role of women is sometimes an issue. But they don't have conflicts on any of the other axis.
So what you do, when you create your characters, is keep them as closely aligned as possible. Please note that this doesn't mean that you're going to create characters who are alike. The axis has nothing to do with personality. Rob is a serious introvert to the point of pretty much being a hermit. I joke that he is the lovechild of Mr. Darcy and Eeyore. I'm a social butterfly and enjoy traveling and entertaining.
Mom and Dad. Mom is a refined Southern woman. Dad is Doc Brown from Back to the Future with a Southern accent. They are closely aligned on everything except, sometimes, manners. Although, argueably it's Marx Brothers, because Dad thinks it's hillarious to troll Mom by playing stupid about some ettiquette thing. They've been married for 52 years and are still best friends.
Character Relationship Plot Arc
Relationships evolve and change over time. The mistake that people make when they write relationship arcs is that they treat them like a character arc. It's a natural leap to make.
A classic character arc (In the MACE quotient) begins when a character is disastisfied with an aspect of self, and ends when they become comfortable with their self-identity. That means all of the conflicts are designed to keep them dissatisfied with self and full of self-doubt.
If you apply that structure to a relationship, what you wind up with are conflicts that revolve around making the relationship dubious. These are the ones where one character will be offended by what another says or the ones where, if they'd just talk to each other...
You know the ones.
The problem with these is that it means that you are constantly introducing reasons why the relationship itself is a bad idea. It is about interior conflict, which means that the conflicts are between the two characters. Understand?
So treat the relationship like an Event Arc.
In a classic event arc, the status quo is disrupted and your character is trying to achieve a stable status quo. It is an exterior conflict, which means that conflicts are not between the two characters. In this case, the characters totally want to be together but outside forces prevent them from achieving their goals.
These are the conflicts where the kiss is about to happen but then Rodents of Unusual Size or Pirates! or even "I've been fired."
The thing about the event arc, is that you can still get the interpersonal conflicts because your characters relationship to each other is shifting with the status quo. What they are working toward is finding a way to make things work, finding a new stability.
But the value of the relationship itself is never in doubt. In fact, the value of the relationship IS the status quo.
For instance, in Valour and Vanity, Jane and Vincent's love for each other is never in doubt, but their status quo is upended when Jane winds up being the breadwinner. This puts pressure on the relationship because their status quo has changed. That shows up then in the places that their axis are out of alignment. In this case, they had fights on the manners axis around gender roles.
But the question was never "Will this relationship work" but rather "How can we make this relationship work?"