I find that semantics is the biggest problem here. What, exactly, IS a "soul mate"? How do you define "soul"?
From my perspective, it's about love. What some think of as "soul mates" are, more accurately, someone you cannot imagine living life without. You may resonate on many different levels (common interests, attitudes, beliefs, etc.) and/or you may differ significantly, which can only make things more interesting for both of you! But you simply feel a need to have them in your life. An urgent, undeniable, visceral need. When that is mutual, then we may use the tag, but the reality is that it's all about love.
You know who mine is.