Why diatonic chords and not always major, always minor, etc?
Why is it important to stay within the key, i.e., to use diatonic chords (minus modulation and key changes)? My intuition is that if the song is supposed to be happy, ergo if it's to use major chords, then all chords would end up being major. What's the deeper reality that has made diatonic chords such a staple in music theory, even if some of those chords are diminished or minor? (I've edited out "the thing to look for" in favor of "such a staple in music theory".)