Quotes: One True Love

In the instant that you love someone
In the second that the hammer hits
Reality runs up your spine
And the pieces finally fit
You're all I've ever needed
Baby you're the one
Elton John, The one''

Their bond. No matter what reality I visited, no matter what world I went to, no matter how many times she hurt him and in turn he hurt her back, they remained bonded by an inexplicable force that I later discovered to be destiny. Fate you can change and play with, but destiny? Impossible.
It's not a matter of them being soul mates; it is far simpler than that. They are the same, two different pieces of the same cloth, they are one, and no matter how much the world, their lives, parents and circumstances drive them to hate each other, their bond will always remain the same. They are one.
Rei talking about Shinji and Asuka, Scar Tissue, chapter 12