Pet peeve based on an often occurring experience. I can't believe the number of people who allow their children to run up to an unknown dog. I have a large, well trained German Shepherd but these people don't know how he will react. I also respect that other people are afraid of this and other large breed dogs. Some times I have an opportunity to change someone's opinion when they meet mine. When asked if my dog bites, I respond with, "Yes but only stupid people, and only command".
I feel for people supported by service dogs. No you do not have to explain what your dog does for you. People - the dog is working - leave it alone.