It is incredibly stressful for dogs to stay in a 'shelter' environment. They are very much in a 'jail cell'. They often have to go to the bathroom in their 'cells' which can be very hard on a dog. They experience the barking and shrieking of other dogs which is very stressful.
I have seen dogs in SPCAs and pounds who lose their hair, often have diarrhea.

Dogs need social stimulation with their owners and with other dogs. It is very sad to see the shape of some dogs at the SPCA.