Obviously sometimes a piece of music is so discordant that people cringe when they hear it, but what I don't understand is what makes people prefer one tune over another when both are free from obvious defects. For example:
What makes "Amar pelos dois" (the song that won Eurovision 2017) better than "Beautiful Mess" (the song that placed second)? Does it sound better? They certainly didn't sound very different to me, and I wouldn't have been able to say conclusively that one song is better than the other. I also happen to not like either song in particular, which is baffling since they're good enough to win a prestigious tournament.
What makes Mozart a great composer? Why is he a better composer than you or me? Can we objectively say Mozart is better than Bach, or vice versa (or Taylor Swift for that matter)?
My best guess is that "if people like your music, you're a great composer", but that's argumentum ad populum which in argumentation theory, is fallacious. My next best guess is that judging whether a piece of music is great involves some artistry that only musicians understand, and my question is what is that artistry? Further: if it's possible to tell upon finishing a song that it's not great, why can't the composer improve the song until it is great?