When two lines merge into one, there can be a brief moment when both sides wonder who should go first. You start to move, and the person in the other line moves at the same time.
In Japan, the most natural approach is usually to take turns, one person from each side.
If someone from the other line has just gone ahead, the next person comes from your side. Then the other side goes again. People are not usually trying to determine exactly who joined the line first. It is more about following the flow and making sure one side does not keep moving while the other is left waiting.
It will not always work out perfectly, though. If the other person starts to go first, there is no need to turn it into a contest. If you are not in a hurry, you might simply let them go and think, “Maybe they’re in a rush.” Then you go next.
But how it feels can depend a lot on the other person’s attitude.
If someone genuinely seems to be in a hurry, letting them go may not bother you at all. If they move ahead as though it is obviously their turn, you may feel quite differently. The order matters, but so does the way a person takes that place in the line.
You do not have to compete for the spot. You can be annoyed, let them go, and take your turn next. Being able to do that without showing every bit of irritation on your face may be part of the art of being an adult.
And if the other person says, “Go ahead,” a small nod is enough, and you can simply move forward. There is no need to get stuck in an endless exchange of “No, you first.”