My theory why people go negative before events that COULD make them happy, like an important football game: lack of emotional intelligence. They struggle with unknowns that could abruptly change in how they feel. Assuming disappointment before it actually occurs might render the final effect less "jarring," but as you say, the bad feeling is needlessly extended by negative anticipation. It also blunts enjoyment of a surprise positive outcome while shaking one's faith in how reality is "supposed" to unfold.
This is why sports are great for learning emotional intelligence -- by staying positive against the odds, winning becomes sweeter. You also are better able to bounce back from disappointment to face the next challenge with positive energy that could prove self-fulfilling. When all seems lost, remember 28-3!