Symmetry and pattern. The theory behind working within in a circle is that the boundary can contain that which is within it. It helps us to focus on ourselves and bring the unconscious to our conscious. It is a great way regulate emotions and the nervous system. Don't worry about the psychology behind it...enjoy the creativity.
Experiment with colour and shape .
Whilst symmetry in mandalas is considered an important part of the spiritual healing, I have found that symmetry is not an essential element and simply enjoy working within the confines of a circle.