Circles represent a need to find unity and peace. They can indicate someone who is struggling to draw everything together and make sense of it. It can also indicate a strong intuitive sense that things are coming together to form a whole, whether that refers to relationships, or life in general. Since they're made with round movements, circles indicate a more passive feeling than angles. They're associated with feeling sociable, talkative and friendly, with a desire to be flexible and loving.
To the northern Native American mind, the circle is the sun, the moon and her children (in terms of gender), man and woman. But, we should not get too literal with these definitions. The Native perspective is vastly deep. And so, the circle also embodies a spiritual energy (as all things - sun, moon, etc holds spiritual connotations - and so too would their symbolic expression). Consider the circle symbol meaning in conjunction with the Native medicine wheels. Here we get the sense of the integration of energy and matter - spirit and man, combining for the purpose of greater spiritual understanding and evolution.
Circles were protective emblems to the Celtic mind. Circles were often drawn as protective boundaries, not to be crossed by enemy or evil. Circles were also Celtic symbols of the cosmos - noting in symbol the intangible procession of time.
In Chinese macrocosmic symbology, the circle expresses the shape of heaven, with earth signified by a square. When we see a square inside a circle in Chinese art, it represents the union between heaven and earth. The deeply significant yin yang symbol is circular, and thus speaks to us of encompassing the whole of duality with intent for unified balance.
Whatever the reason, the circles are here for now!