I believe that there are souls that travel through time continually, and there are certain souls that are connected to others. They are forever searching for one another, and in some way they are always connected. Maybe by the thinnest of strands, but always connected. Always searching for one another so that they can try over and over to love each other.
Sometimes these souls find each other in their life forms at the wrong time, or in the wrong place, and try as they might it doesn't work out. Maybe they come together with such a difference in lifestyle or age that it makes it impossible for them to live and love together. So they walk away, even though in their heart they know they belong together. But the thread still remains connected, waiting for their next time, with the hope still alive that in that next time it will finally work out.
I believe that's why when you meet a certain person you just know. It's like you have known them forever, because in effect you have. You feel them in your heart, and you know what they feel, even when you're not together. It's the magic of the soul, and when two souls can finally become one it becomes destiny fulfilled. And that I believe is a beautiful thing.