I would be very interesting to hear the real answer to this, if there is one.
Good Question Willem.
MJ

I think we are in the same direction, what we know is when there is a chord progression(example key Cmaj) and you strum it, an other player can improvise a lead(or lick) with the notes of that key and this is then not out of key(or tune),so when we turn this around, i mean,make a melody with the notes from that key then you can put any chord out that key under this melody notes,but minor chords works different then major,so how we decide to make a chordprogression?
ps philosophize woth your own knowledge.!