Casanova is right, and yet...
My partner is in a very real way a part of me; I don't mean to sound maudlin, but we are very much intertwined. We will never part, but if he would die before me, I would definitely feel incomplete, like part of me is gone.
At the risk of offending those more religiously committed than I, it's kind of like God and the Trinity: three, yet one. It's a mystery.