Developed as a guard dog, the Doberman has an innate ability not only to protect his family but also to anticipate danger and threats. Because he's so smart, he's not often wrong, but if the Doberman isn't socialized and trained to behave appropriately around strangers, he may show excessive suspicion of guests in your home – suspicion that can turn into aggression.
One of the key phrases there is "lives with his family." While some Dobermans are raised successfully in kennel situations, these are working dogs that have demanding and interesting tasks to do that give them the exercise and mental stimulation the dogs need.