Just general life experience of every single person I’ve heard claiming things like “I only wash my hair / shower / brush my teeth / etc. every x days and it’s totally fine, doing it more often is bad for whatever” not being aware of the fact that they stink, and their friends & family being too polite to tell them, hoping it’s just a fad they’ll grow out of. A person usually isn’t able to smell their own BO the way others are.