I once asked my dad why he liked The Rolling Stones so much. His answer was that they understood the need for highs and lows in a song.
It is hardwired deep in your nervous system. The senses that we humans have developed over millennia of evolution – touch, smell, sight, hearing – are made up of nerve cells linking to our brain. These nerve cells respond better to a sudden change than they do to repeated stimuli.
What does this mean? It depends on the situation.
For Mick Jagger and the boys it means that their songs have light and shade. Quiet verses and soaring choruses. Jagger will whisper and then he will growl and roar.
For emergency response vehicles it means the loud sirens are designed to be varied, sharp and with many different patterns so as to be noticed over the noise of everyday traffic. This contrast works better than one continuous noise which is easily filtered out by the human ear.
For creative people, I think it means that if a piece of art is not getting the desired response, then one of the first things to assess is the use of contrast – light and shade, highs and lows.
Contrast = interesting to humans.