Why do major chords sound happy and minor chords sad? This is a heavily debated question, but essentially it seems that tradition and culture is a big part of it.
However there is more to it than that. Because when you play a note, let’s say low C, you are actually hearing the harmonic overtone series (harmonics).
In this case: C, C, G, C, E (the first 5 notes of this overtone series). As you can see, the major 3rd is present naturally early in the overtone series.
The minor 3rd is also present, but much higher up in the harmonic series, making it much weaker.
The end result of this harmonic series leads to the fact that the major 3rd has a stronger presence in the natural acoustic harmonic series vs the minor 3rd. You can therefore argue that the major 3rd has a stronger natural presence in nature, and therefore feels more natural to us.
The opposite of course then being that the minor 3rd is less natural. The argument here is that the minor chord is perceived by us subconsciously as if there is something “wrong” in the balance of nature. Therefore it makes us feel sad or emotional, since it messes with our natural energy vibes.
So the major sound seems to be natural acoustics of nature itself, and thus we feel more “at ease” = happy state of mind. =)
Yeah I know, this is all very speculative, but culture and tradition + the natural harmonic overtone series, are among the only real facts to support why the minor chords sound sad, and the major chords sound happy.