If you constantly see the negative and expect the worst then you're more likely to be proved right over time, and protect yourself from disappointment.
Having been involved in elite competitive competition and then later coaching too, I can say from experience you have to get these people as far away from you as possible otherwise their persistent negativity will drag you down.
Optimism and pushing for better performances in a positive way is more difficult, especially when things don't go your way, but its the most likely approach to work. That's partly why sports psychology is employed so widely now, to stay positive and keep thoughts away from destructive negativity.