Why do people spit in public?

Why on earth do people spit all over the place? It is rude, disgusting, and illegal! What is the motivation behind it!? What compels them or you to spit on the sidewalk? Do you hate the government and therefore spit protest onto their pavement? Do these "spitters" feel that their spit is like sweet sweet nectar that everyone wants to step in? Is it a part of a sick fantasy of bodily fluids? Whatever it is, these people have no shame. Please answer, I am very confused about spitting on sidewalks!