Predisposed For Pop by Andrew Sullivan

Evolution may explain why we get songs stuck in our head:

Modern humans have been around for some 200,000 years, but written language may have been invented only around 5,000 years ago, Levitin says. So through much of human history people memorised important information through songs. That practice continues today in cultures with strong oral traditions. [Daniel Levitin, an expert in the neuroscience of music] says the combination of rhythm, rhyme, and melody provides reinforcing cues that make songs easier to remember than words alone.

Cari Nierenberg recently identified some other possible causes:

The most common one was music exposure, either recently hearing a tune or repeatedly hearing it. A second reason was memory triggers, meaning that seeing a particular person or word, hearing a specific beat, or being in a certain situation reminds you of a song. The third reason for earworms [is] your emotional frame of mind, or “affective states.”  Feeling stressed, surprised or happy when you hear a song may make it stick in your head. And a fourth cause was “low attention states.”  A wandering mind, whether from daydreaming or dreams at night, can set off this involuntary musical imagery.