system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does interpretations of symphonies (eg Beethoven's 5th) come about? Do the composers write down what their music means and what they were thinking while composing them, or do musicologists just make up something for what they think the music means?
gpt
First off, anybody can have an "interpretation" of a piece of music, based on whatever they think it's about. But usually it's what the composer says it means.

Fortunately, composers like to talk about what their music means. For composers who have died, we can sometimes find out from their diaries and letters. Sometimes they explained the meaning when the music was first published, and there are some pieces where we can only guess.

These days it's much easier. A composer may write a little explanation that's put in the printed music for the musicians to read, so they understand what the music is supposed to "do". Or a composer may write program notes to go with a recording. Or they'll talk about it in an interview.

As for "guessing", certain things in music tend to have certain meanings. It's like how we think of blue as a calm color, or red as violent or passionate. For instance, Beethoven's Fifth starts in a minor key and ends in a major key: it goes from "sad" to "happy". It isn't a perfect guess, but it's based on evidence.