I had a great conversation with a mentor who challenged me to keep increasing the level of positivity in my leadership. He reminded me that people take on the characteristics of their leaders, and that when a leader is negative or pessimistic people pick up on that and their subsequent performance declines.
I have heard it said that the simplest definition of leadership is this: leadership = influence. Negative/pessimistic leaders don't have much influence, at least not the kind of influence that leads to great results. Positive leaders inspire and call people to achieve things they never could have otherwise. There is an appropriate time for realism and struggling, but that time isn't when a leader is out in front of their people.
Be a leader. Be positive.