The major triad is considered the foundation of tonal music, its privileged position owed to its presence in the harmonic series of acoustics. The minor triad lacks this acoustic foundation, which led to it being treated as less stable, and even pieces in a minor key usually ended on the major form of the same triad.

From the late 18th century onwards, major was paired with joy and minor with sorrow, and composers could play with these associations. 

A lecture by Marina Frolova-Walker

The transcript and downloadable versions of the lecture are available from the Gresham College website:
https://www.gresham.ac.uk/watch-now/triads

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