That really sounds like the computer would have to understand you and your shifting moods better than most people could.

Which might actually be quite possible. At the very least, I'm sure machine learning can have a better understanding of your general musical taste than most of your friends do.

That's a pretty exciting prospect then, and I could imagine even a mediocre implementation being almost catastrophically popular.

Spotify seems to implement this with custom play lists generated for you daily, with those play lists broken down by different styles/moods

Personally, I find that Spotify doesn't "get" me in that way, not really. Maybe it hits the broad center of a normal distribution, but I think a lot of people lie outside of that. Even then, surely the challenge of curating existing music which is heavily tagged is much less than composing it with a mood or taste in mind. Just my gut feeling that.