Nature has its own way of maintaining equilibrium within itself! or should I say, amusing itself? :D
Some people, for some reason decide to pollute the nature, releasing toxic waste and disturbing the wildlife, greenery and ecological niche! Now some people see this, feel that its their responsibility to save the nature and set out to do so, fighting against those who polluted the nature. Some might even feel guilt that poor innocent wildlife and humans have to suffer because of few people's activities. Now there's another group of people who don't bother about nature and live their own lives (I belong to this group!), polluting very less and also not planting very much.
Same analogy can be applied to helping homeless, poor, unemployed and saving civilizations! Some destroy, knowingly and unknowingly, and some try to save, knowingly!
Could it be that when our world is about to reach a perfect state of equilibrium, the nature itself triggers something in someone's brain that makes him/her do something to disturb that equilibrium. Some then join this person in disruption and some fight against it... nature lets these two groups sort it out and when fun is over and equilibrium reached again, nature plots something else... nature has its own way of having fun I guess! :D