Why can’t you keep a dog with a pinned tail?

A dog with a pinned tail is timid and cowardly. In a wolf pack, when a wolf with a higher status passes by, the wolf with a lower status will tuck its tail. Taller wolves will release their tails only after they have gone far away. This is related to status. Dogs tuck their tails to cover up the smell of their anal glands, while dogs with high status and confidence will hold their tails high to let other dogs smell the smell of their anal glands. This kind of dog will not get close to humans to please them, and will run away when threatened.