What is the theme in Mother Night by Kurt Vonnegut?

Love is another theme in the book. Out of all the things that the narrator pretended to be in this world, he never pretended about love. Love was the only real thing. He knew it his love for Helga was true. When Resi pretended to be Helga and fooled him, he was willing to overlook that and forgive her because he felt her love was real. After the narrator found out that Resi was a Russian spy sent there to bring him to Moscow, he felt the love was nothing.