i was surprised that when i moved to canada, the sign of ok is check mark instead of circle, so i think it is japanese special tradition that the circle is a symbol of perfection. Also, it associates with the ring, and the ring represents the everlasting promise because there is no end in the ring.
many natural substances are shaped circle such as the moon, the sun, the water drop and the earth. i think the circle could be the symbol of anything.
