When you go to your local grocery store there’s a pattern. You go down your usual aisles, grab your bread, your tea, that thing you know you’re not supposed to eat but do anyway because it is delicious and you have no self control.
And then, when you head to the register, something happens. You get in line. Right behind the person who was at the register right before you. You don’t know them, you don’t technically owe them anything. But you let them check out before you because they got in line first. And you can assume, most of the time, that the stranger behind you will do the same for you.