Different things for different foods. As a kid, I always ate ketchup on scrambled eggs and also on hotdogs. I hated mustard. My mom used to make homemade salad dressing by mixing half ketchup and half mayo, which is basically thousand island without the pickle relish.
As an adult, I like different varieties of mustard on sandwiches, but never on hamburgers. Due to regional tastes, the NYC area is apparently the only place in the US where McDonald's and other fast food burgers do not come with mustard.