It's something that people want to think is true, I think, because it makes them feel like they have more power/control over reality than they really do. The truth of things, of course, is that the universe is cold and indifferent, and what we can do about anything has limits.
OTOH, cynicism can be as bad as thinking that a positive attitude will magically make life better. It can cause lost opportunities, and a sense that things are much worse or more unchangable than they really are. It's also a trap and a bad approach.
I guess the thing is trying to find the balance between over-optimism and over-cynicism that most accurately reflects how the world works, so that one can make the best progress with least mistakes.