I guess I have a glare sort of resting face. My mom used to actually try to shame me and yell at me for looking so miserable all the time as a kid/teenager, and still does it at times today. I don't know how to make my face different!
Additionally, a few times a year I have some stranger who will stop and say something like, "Smile!" to me, and a couple of cheeseballs who will say, "You dropped your smile!" while pointing to the ground. This has been done to me several times, and I always assume it's because I have an unhappy look on my face.