Music is just layered simple patterns and our brains LOVE IT.
Sound is pressure waves, musical notes are a specific pattern of pressure waves. Melodies are repeated musical notes. Songs are repeated melodies following standard structure.
Our brains love trying to decode and parse all these overlapping patterns.
Maybe not really a shower thought and more wild speculation.
I think it’s also about the surprise of something violating the pattern. That’s why jokes are entertaining too. When crafting a joke, you need to build some expectations and then break them all of a sudden. Music has patterns and moments that break those patterns to an extent, so why wouldn’t the same thing apply here?
Music also has this borderline magical property of manipulating human emotions. People listen to sad music to feel sad or angry music to feel angry and so on. I think that sort of thing is another reason why music is so popular.