I don't really agree with the concept of a "Soul Mate", it's all a bit wishy washy, Mr(s) Right is out there, find your Prince(ss) Charming and live happily ever after, fated to be, etc, it makes me sick.
However, I have met a few people over the years that I will be glad to know for the rest of my life. My ex-girlfriend, now just a good friend is one of these people, and I have a male friend that kind of fits this description too. We think the same way, can finish each others sentences, understand each other's quirky humour, like the same music, movies, books, outdoor activities etc...
It's just a compatible personality thing IMHO, I can get along well with almost anyone, but there are a few people in particular who I really click with.