People overestimate the likelihood of outcomes they find desirable and underestimate undesirable ones.
Judgement research, and distinct from optimism bias: this concerns probability estimates of external events, not of one's own comparative standing.
In practice
- A supporter asked the chance of their team winning gives a higher number than a neutral does, for the same fixture.
- A team forecasting a favourable regulatory outcome rates it more likely than the same team would rate an equally probable unfavourable one.