Truth is that positive thinking is easier said than done. Because we are human and that we are basically sensual. We are influenced by the things around us and our decisions fall in these cracks (many times more often). We feel and act on our emotions by how our "world" revolves and resolves with it.
In the process of becoming, positive thinking instead becomes a hindrance. We tend to lose hope. We get defensive. At the end of the day, pessimism seeps and creeps and in our system we slowly become cynics.
It's surprising that things come easier this way.
Because negative thinking is much easier to do.
It's easier to hate. It's easier to criticize. It's easier to get angry at people you can't seem to forgive and reconcile with. It's easier to get pissed. It's easier to dwell in the past. It's easier to not forget. If it wasn't, this world would've been a perfect place to live would it?
So when can pessimism be useful?
I think pessimism works best when we use it in risk management planning or simply put finding solutions to problems. It works worse if it's a personal issue. The former helps - accountants and lawyers call it professional skepticism. Negative thinking works when taken as a tool to cross borders to hopefully understand what lies beyond.
The latter could just be a byproduct of selfish egos.
It pays to know how it should work for you.