This brings up a key concept, namely the inverted U. The complete absence of stress is aversively boring. Moderate, transient stress is wonderful--various aspects of brain function are enhanced; glucocorticoid levels in that range enhance dopamine release; rats work at pressing levers in order to be infused with just the right amount of glucocorticoids. And as stress becomes more severe and prolonged, those good effects disappear (with, of course, dramatic individual differences as to where the transition from stress as stimulatory to overstimulatory occurs; one person's nightmare is another's hobby).