From 《The Music of Man》 (9)

That sense of tension and relaxation is in fact built directly into Western harmony, in the alternation between the major and minor. The major chord is the first and only pure one we encounter in natural overtones, and the major is generally regarded as happy, the minor as sad. I think this has to do with the fact that the minor has dropped the natural interval of the middle note in the chord by an artificial semitone. In C major the natural middle note is E, and in C minor that note becomes E-flat, where it conflicts with the implied E. The conflict is not so strong as to upset the strength of the basic three-note chord, but it is disturbing to the natural overtone series bey9ond. And what is sadness? It is a division, a sense that things are not right, that you want what you cannot have or lament what you have not done. You bewail the loss of something, you are not at one with yourself or the world. In that minor chord, that loss is the purity of the middle note, which wants to rise, to become major, to unite again with its natural series.