There's a popular idea that there are pairs of people who are perfectly matched with each other. They are supposed to be together. God, fate, or some other supernatural force wants them to be together. People say that these people are "soul mates".
You're my wife, my best friend, and my soul mate. I'm so blessed to have found you. Happy anniversary, sweetheart!
Usually "soul mates" are a romantic pair. However, sometimes people say that a non-romantic pair of friends are "soul mates".