There's a Song in my Head

Why do songs get stuck in our heads, playing over and over? For months I’ve had the tracks of Avenue Q appearing spontaneously, then at the weekend John & Rich had ‘A New World’ on one of their playlists and that became the new theme-to-my-day and now, after last night’s rehearsal, I’ve got ‘I Just Wanna Dance’ from Jerry Springer The Opera going round and round behind my eyes. Are there any psychologists or neuroscientists out there who can tell me why these things keep sticking in my head??