Why do you assume things will turn out fine for you?
You quietly bet on the good ending, even when the numbers say otherwise.
On average, people expect good things to happen to them more than reality warrants, and bad things less. The pull is real but moderated - it is an average tilt, not a rule, and it weakens when you are low, well-informed, or in a sour mood. One reason it sticks: the brain takes in good news about your future more easily than bad news.
You tell yourself the 2-week side project takes 2 weeks, that you will not be the one who gets food poisoning from the street stall, and that this lottery ticket feels different. Three small bets on the rosy ending.
Your gut forecast leans bright, so for anything that matters, check it against the base rate before you commit.
It is why deadlines slip and budgets blow up - planning off the rosy guess instead of the real odds. Add a buffer on purpose.
Optimism bias = good news sticks, bad news slides off.
Learn the idea and practice English at the same time.