musicology

There are two principle chords in music.  Major and Minor.  Major chords are traditionally associated with happiness, joy and have a bright optimistic tonality about them.  Minor chords on the other hand have a darker feel, conveying a sense of sadness, grief, tragedy or other manifestations of sombriety.

The question is, what is it exactly within these chords that is responsible for such a dichotomy of emotions?  What makes 'sad' music so sad?