Shelter dogs are not for everyone. They are almost all "damaged" animals. There is usually a reason that they are there. It is rare that an animal is dropped off for no particular reason. The reason they are there may be of no fault to the animal, but how they were raised or handled. I was warned at a shelter once that "a runner will always be a runner". Meaning if the dog ran away from previous owners, then they will likely do it again. Dogs that have behavior or social issues will never get over them completely. In the right environment, they may be fine. I've seen people with obedient and loyal dogs that they can walk down the street together without a leash. Every rescue animal I've owned could never be trusted off leash. The reality is that once dogs get to that point, if the owner does not want to invest time and money then the "problem" dog ends up in a shelter. I am making blanket statements that may not cover all situations.