The way I figure it, the rings work on willpower, and technically everybody has willpower to a certain degree, so technically everyone can make the ring do something. But to use it effectively requires willpower very much above the average. Just because you can run doesn't mean you can run a marathon. Just because you can drive a car. As for overcoming fear, I always just took that as a benchmark for the willpower requirement. Like, if you have the will to push through great fear, then you'll be able to utilise the ring effectively. If not, sure, slip the thing on, but it wont do you any good.