I believe in the concept of soul mates, there's this 'inner knowing' when we meet them and it just clicked on multiple levels. I've crossed paths with one a couple of years ago but later down the road our paths separated again. It's indeed a bit hard to put into words and it can only be understood through feeling.
From what I understand it is that soulmates can come in to our lives for brief periods also and then walk out. True, these things are difficult to put in words, it can just be felt and sometimes the pain is too much when they walk out.