Of the various elements of fiction, theme is probably the most difficult to discuss. This is why there is a great difference between reading a story that explores the theme of the difficulty of love and reading the message "Love is 
difficult." in a fortune cookie. The difference is that the mere message does not allow us to experience its truth – we either accept it or we don’t.  

    A theme is the understanding that the author seeks to communicate through the work. It gives the work its purpose and has a great deal to do with the way the whole is constructed. By theme here we mean not "message" but the general subject. Theme is at once the beginning and end of our search. The search itself is what gives value to our engagement with literature.  

References:  
Understanding Fiction; Robert Penn Warden; Appleton-Century-Crafts Inc, New York 1943  
The Art of Fiction; John Gardner, Vintage Books, New York; 1933