In Japan, I think that circle means human relationship. One good example is friends. Many people have heard the words "widen the circle of friends", which mean make new friends. Then, human relationship is widen like an annual ring of wood. And like there is no ending on circle, that relationship will not end so easily.
When we look around, there are a lot of things shaped circle, so we human beings unconsciously think circle as something that gives us special power and something very important for us.
