Believing that the worst will happen fundamentally limits your available perspectives for every situation you encounter. The inability to imagine a brighter path forward stunts your ability to improve the status quo.
No great advancement in the great story of human endeavour has been launched on the back of boundless pessimism. Certainly many (if not most) adventures have been sparked by less than benevolent intentions, but the actual artefacts of success are the result of minds that fundamentally believed in their owners’ abilities to enhance the world around them or achieve some goal.
Further, nobody is inspired by the belief that everything is awful. That something will fail.
“I’m not a pessimist, I’m a realist” cry miserable bastards everywhere.
Your grasp of reality doesn’t impact your outlook. Your belief in how reality will play out (regardless of how many outcomes you may foresee) is that which defines you as a pessimist or optimist.
Being an optimist doesn’t mean you’re living in a fantasy. It doesn’t mean you’re unburdened by the realities of daily life. It means that you accept reality as you find it and try to find the positive elements of each and every encounter you might have.