My wife used to worry quite a bit. When we're driving together I often talk about things we'll see and explain the defensive cycling measures I take to identify and avoid those situations. That seemed to alleviate some of her concern, as she knows I'm not just some yahoo out there playing in traffic.
Then two years ago at the age of 43 I was diagnosed with a genetic heart condition that if left uncorrected will eventually kill me. My wife asked the Dr how it could have gone this long without it causing me any problems, and the Dr flat out told her that if I wasn't in such good shape from exercise that I probably wouldn't have lasted this long. Since that time not only does she not worry about me riding, she's encouraging me to ride whenever possible.