The more someone complains, the less accurate their view of reality is. Because usually you're complaining about reality not meeting your expectations.
But that's not reality's fault. It's your fault that reality didn't meet your expectations.
Obviously stuff can happen in the world that's unfair, and that sucks, and you should point it out. But the complaining bit to yourself? Reality's 1,000 and 0. Undefeated. Total knockout.
If you've got a friend who's always complaining, I don't think they understand how the world works. They straight up can't. Because if they did, their expectations would be closer to it, and they wouldn't be complaining as much.