Why Java inner classes require "final" outer instance variables?

final JTextField jtfContent = new JTextField();
btnOK.addActionListener(new java.awt.event.ActionListener(){
    public void actionPerformed(java.awt.event.ActionEvent event){
        jtfContent.setText("I am OK");
    }
} );

If I omit final, I see the error "Cannot refer to a non-final variable jtfContent inside an inner class defined in a different method".

Why must an anonymous inner class require the outer classes instance variable to be final in order to access it?