...somebody else would do. Or so says Tim Minchin.
Blasphemy! you might say. Surely, there's something special about the person with whom you're destined to be. Your (future) spouse is unique, irreplaceable, and somehow specifically fated to be with you and only you. Or so says the myth of the soulmate.
One of the most harmful myths ever to crop up in romantic lore.
I view the notion of "soulmate" as tremendously harmful to relationships because it perpetuates the notion that somehow, there's only one person who's perfect for you. And when even minor problems crop up, it becomes clear that the relationship (and hence your theoretical soulmate) isn't perfect, so your "true" soulmate must be somewhere else. Relationship after broken relationship is left in the pursuit of the perfect person. Unfortunately, the perfect is the enemy of the good.
In reality, there are a number of people with whom you could be successfully married. Now, the vagaries of circumstance severely limit that pool of available people, and the process of dating is largely about encountering someone with whom you share compatible values, goals, and coping mechanisms.
However, the thing that makes married love feel so unique is the particular series of trials and tribulations you face together and surmount, along with the joys and triumphs you achieve. As you build a life together, you weave a story that marks your lives together, incorporating the high and low points into a narrative tapestry that (in hindsight) looks like it was fated to be constructed that way.
However, this process happens in any long-term relationship. As long as you've got someone compatible enough who you'll enjoy through the long haul and who is a good support for you, you'll eventually make your own story that feels as if it was written between soulmates.