Yorkshire terriers, like most purebreds, are susceptible to several behavioral conditions. Because of their natural in-charge attitude, you'll have to be a firm authority in order to keep your dog under control. They will run your household if you let them, and can become manipulative and demanding if they're used to getting their way. However, they can just as easily be perfectly wonderful companions. The way you treat them, and what you expect of them, has a significant effect on how they behave.