Why diatonic chords and not always major, always minor, etc?

Question

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".)

Answer 1

There are tunes that use only major chords; they are usually very simple songs, but some of these are quite famous, such as these four 3-chord songs: https://www.youtube.com/watch?v=zi6CRi0SXLM

However, the more musically interesting songs will have one or more minor chords, to provide contrast and variety. John Denver gets away with having only one minor chord (appearing as the second chord in the verse and the third chord in the refrain) in his "Country Roads" - https://www.youtube.com/watch?v=1vrEljMfXYo

Answer 2

Hello!

If you played a song with all major or minor chords, there would never be a strong tonal center present. Minor and diminished chords are needed in major keys to establish the tonal center and to create a strong drive toward it. Without them, there would lack a desire to resolve the sound to a certain pitch. They also are great for creating high and low points in the composition, creating tension to resolve, and when you understand the theory behind harmonic progression you can have control over the direction you want the piece to go.

I hope that helped answer your question!