When people think of soul mates they think of happily ever after marriages with lots of love. And this is one type of soul mate. The most sought after.
However when we die, I believe we go to spirit (or heaven, summer lands, whatever you wish to call it we go to a spiritual place). Once there I believe we are surrounded by friends, family and other important people from our lives. I say lives because I do believe we have many of them. I also believe that we reincarnate with these spirits that are close to us. Those spirits are soul mates. They can be your best friend, your family, your coworkers, and your lovers.
I absolutely believe my husband (Lance) is my soul mate. He completes me and I complete him. And as the above post says about love at first sight, when our eyes met, that was it. He was the only one for me. I think somewhere deep, our souls recognize the person we are meant to be with. Some may ignore it or maybe not everyone is connected enough to feel it. But its there.