Why do we keep our earth clean by walking, ride a bus or riden with someone else

Because if we do not then the Earth will go through a lot of issues in about 10 years, some scientists studied it.

fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled, via osmoregulation and behavior, such that the concentrations of electrolytes (salts in solution) in the various body fluids are kept within healthy ranges. the core principle of fluid balance is that the amount of water lost from the body must equal the amount of water taken in.