He knew that I love you also means I love you more than anyone loves you, or has loved you, or will love you, and also, I love you in a way that no one loves you, or has loved you, or will love you, and also, I love you in a way that I love no one else, and never have loved anyone else, and never will love anyone else. He knew that it is, by love's definition, impossible to love two people.
