I think it is just a bad habit and just a touch of human nature. For example, I think people might be more likely to complain to a restaurant manager about a bad experience than to ask to speak to the manager to tell them how wonderful everything was.
There is an app I've seen my daughter and friends read called FML, (blank) my life. I hate it. I've told her someone should make PML for praise my life. It is just a matter of turning the focus around; but like a muscle that needs to be exercised, focusing on the good is easier the more and more you do it.