For a singer I think it's the perception that a lower third note will be easier, and if off pitch by a little not so noticeable. (think blues, which again implies sad)
Someone else mentioned the overtones/harmonics
The major really needs to be closer to spot on pitch in order to establish said effect.
In my experience singers are more likely to accidentally err on the side of flat which is less forgiving in a major key (just my thoughts from how I see/hear things, YMMV)


I do not work here, but the benefits are still awesome
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