I believe they believe they are doing the world a great service. Contributing nothing of their own because they spend all their waking hours reading/listening to the contributions of others to find fault with it so they can correct it. It is what it is. They are whom they are. I guess it keeps them off the streets and out of bars. Mebbe not.
With regard to paid critics who earn a living criticizing do you pay any attention them? If they thumbs down a movie do you not see it? If they thumbs down a restaurant do you not check it out for yourself? Do you live your life following the opinions of critics because you think it will save you from wasting your time and money? Why?
Ever see a movie the critics panned that you loved? How about a movie the critics loved and waxed poetic about that you thought was stupid mundane sophomoric and a waste of time? Why would anyone substitute a stranger's opinion for his/her own? I don't get it.