Interestingly there is a body of research that suggests enjoyment of music comes from having exactly one of two things, never both:

Familiarity and predictability

If it's neither familiar nor predictable, it is inscrutable and therefore discomforting to listen to

If it is both familiar and predictable it is boring

If it's familiar but unpredictable, it feels like a journey through known emotions

If it's predictable but unfamiliar it feels like 'logical discovery' and is fun and satisfying

A bit reductive but I love this idea

Prog music lovers in shambles.

We like music for the same reason we like games, stories and successfully accomplishing tasks.

It's the vibe that it evokes.

Patterns evoke a vibe too.

Vibe = poetry emotion energy.

It's a sensation like sight, sound, smell etc. Just a different kind.

Songs are repeated melodies following standard structure.

Plenty of music isn't. But maybe the joy there is that it's not as formulaic?

Musician here. This is definitely true, BUT interest can also come from subversion of those expectations. Can be seen in prog music and math rock (subversion of musical forms), funk (subversion of rhythmic expectation with lots of upbeats and short notes), etc

As a speculation it's really pretty good. Many years ago there was a Scientific American article about why people like music. It was long and complicated but the tl;dr would go something like:

Well-liked music of any genre tends to contain fractal patterns. Doesn't matter if it's jazz, classical, rock or whatever. If you probe our peripheral nervous system you get a lot of white noise, but the closer you get to the central nervous system the more fractal the signal becomes, as if our nervous system is filtering out the noise and letting the fractal part of our perceptions get through to our brains. This makes it very likely that our thoughts and memories are fractal patterns, which means that on a purely mathematical level there could be similarities between patterns that encode ideas that aren't related by context - for example, when a piece of music makes you think of the ocean, or flying birds, or the big city, it's probably because the music's pattern and the idea's pattern in your head are mathematically similar.

I strongly believe that our brains are fundamentally just prediction machines. We strive for a specific level of controlled novelty, but for the most part 'understanding' (i.e. being able to predict) the world around us is the goal. We get boredom to push us beyond getting too comfortable and simply sitting in the already familiar, and one of the biggest pleasures in life is the 'aha' moment when understanding finally clicks in place and we feel we can predict something novel.

I feel this is also why LLMs (ChatGPT etc.) can be so effective working with language, and why they occasionally seem to behave so humanlike -- The fundamental mechanism is essentially the same as our brains, if massively more limited. Animal brains continuously adapt to predict sensory input (and to an extent their own output), while LLMs learn to predict a sequence of text tokens during a restricted training period.

It also seems to me the strongest example of this kind of prediction in animals is the noticing (and wariness) when something feels 'off' about the environment around us. We can easily sense specific kinds of small changes to our surroundings that signify potential danger, even in seemingly unpredictable natural environments. From an evolutionary perspective this also seems like the most immediately beneficial aspect of this kind of predictive capability. Interstingly, this kind of prediction seems to happen even on the level of individual neurons. As predictive capability improves, it also necessitates an increasingly deep ability to model the world around us, leading to deeper cognition.

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.

Just like in any form of media, we enjoy a balance between familiarity and novelty. Different people have a different sweet spot for that balance, which is where we get different genres and styles

Explain why Jazz is so fucking awesome then.

More wild speculation:

1. Patterns make brain release happiness hormones, because it rewards itself for correctly guessing how the pattern continues. The rewards stop when the pattern is overly repetitive, because the brain recognizes that this isn't a challenge to guess correctly. And then it also quickly becomes not worth paying attention to, because if it's not going to change, it's irrelevant to our continued survival and all.
2. This applies to anything where humans perceive beauty. A painting with a consistent color scheme or technique is nice, but just a blue canvas is pretty boring. A poem that rhymes is nice, but if it rhymes twice or thrice by using precisely the same device, then it doesn't entice.

And then you listen to Six Degrees of Inner Turbulence (275 time signature changes)

I think it is way more deep than that.

I think this is getting it backwards. Here I’ll go (warning, evopsych style speculation follows):

Our brains are great pattern recognizers because it makes us better at learning music (and other structured forms such as poetry). Music is older than all the civilizations on earth. We learn music because it’s an incredibly powerful aid to memorization. Memorization and oral recitation is the oldest form of cultural transmission we have.

Culture is the secret of our success as a species. It’s the original problem solver that gave us so many tools and techniques to survive on every continent on the planet (except Antarctica of course). Culture is the reason we learned to prepare so many foods which would have been poisonous otherwise (such as cassava).

For a long time lots of European music was mostly thorough-composed, where there was little to no repitition. Madrigals (the popular music of the renaissance) were mostly like this, the melody would follow it's own journey with no chorus / verse or other repetitive structure. I might be remembering wrong, but I think it was early baroque and Monteverdi's Orfeo that popularised repeating structures, and turns out people love them. If you back and listen to some madrigals, it's a very different approach to music. (also, there was folks music and all sorts of other traditions, which used more repeating patterns, that seem more familiar to us.)

Music showers my brain!

We like music because it acts like a pleasurable drug. Probably something primitive and hormonal that is connected to mating (like birds do with bird song).

Ya let’s not pretend why humans enjoy music. No one can explain it.

Frankly, this kind of reductive deconstruction of human experience is a huge part of what's going wrong with society. Music is created by people who dedicate themselves to exploring the world we experience, and discovering nuance which evokes feelings that can not be simply explained, betraying the existence of deeper layers of human experience than anything directly available to cold analysis.

Certainly there are then those who copy the form of works created by such artists, who actually do treat music that way, but the result is like the output of a LLM, and repetition of such a style is like training AI on it's own data, progressively degrading what was once there.