Eclipse
It is said that opposites attract one another. Because that way, they complete one another. Each side has something that the other side doesn't and vice versa. Together they make one body. They balance each other so that the body can have perfect symmetry.
The same goes for soulmates. Although they have many similarities, their personalities are very often opposed to one another, like day and night or sun and a moon on the same planet.
If one is outgoing, the other is not. If one is shy, the other is not. If one is impulsive, the other is more levelheaded. If one is emotional, the other is reserved, etc.
They need each other to complete each other. To create that symmetry that will balance their existence.