"So, if you played a C major chord to pretty much any person on the planet, they'd say that it sounds "harmonious" (or pleasing, or happy, etc, etc). But now when you want to put chords and melodies in an ordering and make a larger piece called a "song", then that is a much more difficult process, and gets very subjective. At that point, it's not just the chords, it's the lyrics, rhythms, instrumentation, tempo, intensity, any number of other things that goes into a song... so many variables that it's almost impossible to predict how a song will affect a given person."