My background is in music theory, and, as others have commented here, this is a question that there's no clear, agreed-upon answer to yet. Music theory can tell us what composers tend to do - which chords they choose should follow each other, which notes to use when - but it's not as good at telling us why those chords / notes sound good together.

In building Jukedeck, we're applying our own theories about what makes good music. And, like any musical composition, it's an experiment!