Why do minor keys sound sad? Because of the melodic relationships. If you play the white keys on the keyboard from C to C, it’ll have a bright or happy sound. Conversely, if you play from A to A, it’ll have a dark or sad sound.
This happens because the pattern of whole (w) and half (h) steps changes between the minor and major scales. The C major scale has a pattern of w, w, h, w, w, w, h. You can see this on the keyboard diagram below.
The white keys all have black keys in between them – except for E & F and B & C. E & F and B & C are the half steps from the previous pattern.
In contrast, the A minor scale has a pattern of w, h, w, w, h, w, w. Because our ears hear in relative rather than absolute terms, the pattern shift completely changes the sound. Listen to the soundtracks to hear the difference.