According to Osho, pessimism is the mind’s negative lens born from the collapse of shallow optimism: after repeated disappointments, one becomes an ex-optimist who counts thorns and ignores roses, constantly seeking flaws and expecting the worst. It’s an easy, unconscious stance of saying no to life, which can be transcended only by growing awareness beyond both optimism and pessimism.
It’s when getting let down makes you look only for what’s wrong—like seeing only the thorns and missing the rose.