It is frankly quite lovely to read about such a relationship, especially if the story is written well. That is, it is lovely if the reader is carried alongside the relationship. We aren't simply told that the couple is right for each other. We see it through conversations, events, and over time. We "hear it" through fresh and witty dialog. We watch it unfold.
Here's the writing problem. Unless the book is a travelogue, at some point a crisis will need to occur. For romance writers, unfortunately, that crisis--if it is internal (specific to the characters)--can undermine everything that went before.
The couple are right for each other--until a pang of jealousy, a horrible in-law, a misunderstood conversation sends one or the other reeling. How ever will the relationship recover?!
And the thought arises, Are they truly soulmates? It didn't take much to undermine their mutual trust.
In such books, I prefer the travelogue solution. I find myself pleading, Please don't mess up the relationship I love just to get a narrative arc.
Oz by Lily Morton deserves special mention here. Lily Morton is quite good at creating fun relationships that the reader roots for. She has--by necessity--fallen back on the unlikely crisis to wind up a story. But in Oz, she manages to create a climax without sending Oz haring back for London (which I honestly was sure she would do).
*Slight Spoilers*
Morton manages this because although Oz has a sharp tongue and sharper wit, he is also protecting himself, which Silas, his boyfriend, recognizes. Oz has to admit things to himself before he can fully accept that he is Silas's soulmate.
There's an Austen-perfect sequence where Oz and Silas visit London together, and Oz realizes that he isn't the London-city persona that he has taken on (as self-protection). He truly is a country boy at heart. He wants to live in Silas's big, drafty, old house in Cornwall.
In addition, it is psychologically realistic that the one person who could flatten Oz's take-me-as-I-am attitude is Silas's hoity-toity mother. His own mother is so important to him, disappointing Silas's mother is a bit difficult for Oz to take.
So he experiences a moment of uncertainty, but he rallies when he hears Silas's defense. And he doesn't run anywhere. I was extremely relieved since the "I love you--we're perfect partners--but I couldn't possibly be good enough" plot line gets kinda old.
Another solution to the Perfect Couple Who Suddenly Suffers a Problem is a change of scenery.
To be continued...