Devising patterns for each concentric circle is calming.
For me, mandalas are like doodling with a purpose, in concentric circles. It's all about moving pen on paper. Stephanie's freehand work at Flickr inspired me to draw mandalas. Stephanie blogs and documents pen and ink testing at Spiritual Evolution of The Bean.
Explore the Handmade Mandalas group at Flickr. According to The Art Therapy Sourcebook by Cathy A. Malchiodi, "Drawing or painting in a circle is containing, structured, and pleasing. I have personally found that creating spontaneous mandalas lessens anxiety, evokes inner focus, and creates a sense of calm."