This is a nice story about a saint named Shirley who has fostered hundreds of dogs over the years and made a big impact in the rescue community: http://thebark.com/content/discovering-great-forlorn-dane.
Tyra, the Great Dane Shirley rescued was in bad condition from the start. She was a stray and did not want to be captured, even trying to bite while being approached. Once she was at the shelter, she was not a fan favorite and was not likely to make it out of there without Shirley.
After taking Tyra home, Tyra initially wanted nothing to do with the family or the other dogs but slowly started to warm up to them over time. Ultimately, Tyra became true to her great dane form and started being very pouncy, silly, sweet and loving. In fact, she became such a part of the family that Shirley adopted her and takes her to work with her every day. Tyra right now even helps Shirley with other dogs as she keeps them balanced.
The reason we are posting this story is that this is characteristic of many dogs at shelters that rationally do not respond well to being confined and abandoned. They are nervous, have no outlet for their energy, and are depressed. This is especially a problem with some of the more active breeds. There really are no bad dogs at birth just bad owners who, so if you see a dog circling in their cage and depressed do not rule him or her out immediately. Most dogs will eventually come out of their shells. There are some dogs that bite and shelters will not adopt them out, so if the shelters clear the dog think about giving them a second chance.