Recently I had seen a great TED talk by Benjamin Zander in which he says (about Chopin's 4th prelude):
...... The first note is a B. [plays B] The next note is a C. [plays C] The job of the C is to make the B sad [plays BCBCBC], and it does, doesnt it? Composers know that; if they want sad music, they'll just play those two notes. [plays Mozart's 40th symphony transposed to E minor as an example (CBB,CBB,CBBG....)] .........
I'm trying to understand this in the context of music theory, I can think of two reasons:
The composer, by using those notes makes it clear to the listener that it is a minor key (otherwise it would have been B and C#) and minor keys are (usually) perceived as sad.
The B and C form a minor 2nd interval which is dissonant and hence we perceive it as sad.
However I am not so sure about these reasons, here are the objections:
If it is melodic minor instead of natural minor it would have been B and C#, same as the major key, but even the melodic minor is sad.
Even the Major 2nd interval is dissonant yet it is present in the major scale and doesn't sound sad.
So can you please help me and clear my doubts?