It doesn't have to be about distrusting a person as much as distrusting that the person is security aware. E.g., most non-technical people I know have no problem with logging into their Gmail from internet cafes. You may be 100% that your wife would never screw intentionally you over and yet not so eager to share your passwords with her for that reason. Sure, you can teach her not to login from random computers, and then the difference between http:// and https:// and what it means when the browser says that the certificate cannot be verified, but it can turn out to be a quite large project.