The principle is quite simple really: when you think negatively, you act negatively. Then you blame the world when things go south when you didn't even try to go the other way. Same thing with positivity, except reversed.

I've read some drama, though most of the people I follow tend not to indulge in a massively pessimistic behavior. Usually I just tend to ignore that type of stuff, because it doesn't really bring any solutions nor does it put me in a good state of mind.

I agree with the "be the change you want to see". It is ridiculous to try others to change their behavior if you yourself are not going to change. I wouldn't hold such people to a very high standard myself at least.