"Melodies which run through your mind may give the analyst a clue to the secret life of emotions that every one of us lives. In this inward singing, the voice of an unknown self conveys not only passing moods and impulses, but sometimes a disavowed or denied wish, a longing, and a drive we do not like to admit to ourselves. Whatever secret message it carries, the incidental music accompanying our concious thinking is never accidental."
-Theodor Reik, referring to when music gets stuck in your head. I'd love to study this more to see how it works!