Neuroscientists think earworms are a byproduct of the brain's predictive processing system:

• Your auditory cortex constantly predicts what comes next in a melody
• A pattern that is mostly predictable but slightly surprising makes your brain run the prediction loop again and again
• Gaps in the sequence (hearing only the chorus, not the verse) leave the pattern unfinished, so your brain keeps trying to complete it
• It hijacks the phonological loop, the working-memory system you use to hold a phone number in your head

In short, an earworm is your brain rehearsing a musical prediction it finds interesting.