We really don't get to understand /why/ most of the time. It's true.
I've loved him my whole life, and somewhere along the way, that love didn't change but grew. It grew to fill the parts of me that I did not have when I was a child. It grew with every new longing of my body and desire until there was not a piece of me that did not love him. And when I look at him, there is no other feeling in me.