system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do musical chords sound good?

Why do the notes that make up a major chord sound good together and others don't? Why do three random ones sound terrible?
gpt
Acoustics, physiology, and cultural bias.  

Imagine each note on the piano is a square grid, and each grid is large or small depending on how the note is.  Now take two grids that have similar proportions and lay them on top of each other, this set will create shapes that are symmetrical and in alignment with the other grid.  Now imagine that you have two grids that aren't similar sizes, when you overlay these on top of each other the shapes that are created look random, and are definitely not symmetrical.  These grids are very similar to what happens acoustically with "good sounding chords."  Every note has an overtone series on top of it, which is like the grid, and when the notes are pleasing the grids line up in a sort of symmetry. You can see this mathematically as well, a perfect fifth, a very pleasing interval, the foundation of the power chord, is a ratio of 2/3.  But a minor second is a ratio of 11/12 (when tuned justly, which is a whole other thread). 

Our ears also process sound in a specific way based on how it is shaped.  Inside our ear, our hearing cells are kind of similarly laid out like a piano keyboard.  The nerve cells for things that aren't pleasing are close together, and some theorize that the cells are bumping into each other.  This bumping into each other could be unpleasant, but they are still trying to figure this out honestly.  If someone else knows more information about this please correct me! 

Finally cultural bias has a great deal to do with what sounds pleasing and good.  For example, Gamelan music from the south pacific uses a pentatonic scale with two notes that are close together but not in tune for each scale degree.  This scale is called the Pelog scale.  To westerners this music is very dissonant, but to those in the region the music is considered to be extremely beautiful.  Using the first example, this music uses grids that don't line up very symmetrically.  The culture of both regions impacts very much what people interpret to be "good sounds."  

It's a little bit of all three columns at the end of the day.