Why do you root for your side even when the sides were drawn at random?
Flip a coin to split a room into two teams, and people start quietly favoring their own - for no reason at all.
In-group bias is the pull to favor people who share your group, even when that group is arbitrary. In the minimal group experiments, strangers split by nothing more than a coin flip or a fake art preference still handed out more rewards to their own side. The group did not need a history, a rivalry, or any real stake. The label alone was enough.
On day one of a new job they split everyone into the blue team and the red team for an icebreaker. By lunch you already want blue to win the silly quiz, though it was decided by a random draw an hour ago.
Even a meaningless line between us and them is enough to make you tilt toward your own side.
Knowing an arbitrary label can bend your judgment helps you check whether someone earned your trust or just happens to wear your colors.
Same jersey, instant bias - the team can be made up and you still pick it.
Learn the idea and practice English at the same time.