We have a biological predisposition to think negatively- it's a protective device our ancestors used to avoid threats. This default mode of thinking doesn't serve us as well these days in either our personal or professional lives. It leads us to think poorly of ourselves, doubt our abilities, and expect the worst of others. This energy can be toxic to ourselves (both physical and mental health) and those around us.
The good news is research shows we can rewire ourselves to default to positive thoughts- we just have to train our brains.