It's really simple - it's because I have finite time, energy and resources in the world, and I find that negative people and negativity - is a black hole.
It doesn't stop naturally, because it is a habit that has been formed and reinforced over time (read an article I wrote years ago on Stop Complaining).
Yes, in some situations, it is very helpful to:
...but...to some people, complaining has become a habit and part of the person, so what happens is that certain people who has adopted/become negative, has many and much of negative thoughts in their mind that all that comes out of their mouths are mainly negative stuff and complaints for the sake of complaining.
Though I personally don't really like it, I find that I built up quite a tolerance for it such that I can "go through it" like managing a processes, BUT I've come to a realization:
Some people, things and stuff, which are inherently and habitually negative - I no longer want to associate with them or spend resources, energy, emotions and time more than what is minimally required.
Better still, avoid and ignore entirely.
I am moving a little cautious in this direction, but I am certain that when I focus on things that matter, my happiness, business and life will improve manifolds as well.
I want to actively and consciously use my time, gifts, energy, emotions and resources on projects that matter to me.