system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Biologically, what makes a song catchy?

I'm looking at you, Latch by Disclosure.
gpt
Repetition & variation.  Repetition establishes a pattern and then variation creates unexpected surprise in that pattern.  The brain tries to adapt to the pattern but needs to re-listen to the variation to grasp the nature of the unexpected surprise.  The repetition can occur in one or more of the elements of music:  rhythm, melody, structure, background vs. foreground, or multiple elements against each other.