Why can’t the symphony require all music to be happy? Why can’t a human require all her moods to be happy? Art is supposed to represent, reflect, and confirm the fullness of the human experience; art is the means by which we, as a species, communicate with ourselves. To illuminate even a single soul requires evocation of an infinite depth of emotion, not just one emotion but a whole teeming catalog of them, spilling across days and years and lives, common moods popping up like motifs and disappearing. The sounds of life are written in every key; the sights are painted in every color. To deny this is to deny yourself. To deny art which is not happy is to wall off nearly all of your being from contact with the outside world.
I suspect that someone who demands all music be happy has forgotten how to be truly happy.