Why do songs get stuck in my head?
When we listen to a song, it triggers a part of the brain called the auditory cortex. Researchers at Dartmouth University found that when they played part of a familiar song to research subjects, the participants' auditory cortex automatically filled in the rest -- in other words, their brains kept "singing" long after the song had ended [source: Prokhorov].
The only way to "scratch" brain itch is to repeat the song over and over in your mind. Unfortunately, like with mosquito bites, the more you scratch the more you itch, and so on until you're stuck in an unending song cycle.