It's all about your emotional state, remember that spirits and demons embody concepts they come in contact with in the Fade.
If you are a noble mage with good intentions and strong determination, it's likely that you will attract benevolent spirits to aid in your hour of need.
However, if you feel negative emotions like rage, hubris, obsession or despair you will attract demons corresponding to those emotions.
The big difference between Circle Mages and other mages is that they live in an institution which considers magic to be inherently evil and chaotic.
Circle mages are indoctrinated to the idea that their magic is beyond their own control and that they need to be confined to prevent them from being possessed by demons.
Like Solas said, Andrastian mages from the South grow up under the belief that all communicating spirits are demons and that they must fear and reject everything that comes from the Fade.
That belief in itself creates fear, anxiety and insecurity which not only attract demons more easily but also twist the perception of the mage into seeing every spirit in the form of a demon.
If the mage grows in a more Fade friendly environment, chances are that they will be able to differentiate between demons and spirits far easier and that good spirits will help teach them how to defend themselves from the bad.
So far the percentage of mages being possessed by demons amongst Dalish, Chasinds, Avaar and Rivaini appears to be lower than that of circles which reinforces that an oppressive instituion does far more harm than good in preventing the spread of abominations.