Songworms

What is it about songworms? Why can just the slightest mention of certain lyrics cause a person to have a song stuck in their head for a LONG time?

Example: This morning I logged on to Facebook and near the top of my feed an FB friend had a status update that made me think of an Elvis Costello song. This happens to be a song that I know and like and I don't need the iPod to sing in my head. WHAMMO! I got the songworm.
Two hours later, I am still singing the song in my head.

Nothing works. Practiced my Spanish. Read the paper on the bus. Listened to the news. NOTHING.

It could be worse. My cousin (who has three small children) has reported singing the theme song to "Caillou" all day long. At least I like Elvis Costello....