Bonny there are actually lots of wonderful dogs at shelters that have no problems at all. Sometimes wonderful dogs are dropped off because the owner is moving and no longer wants it, sometimes you'll see people turning in their dog because "it doesn't match the couch", or it sheds, or for other silly reason. If you find a good rescue they will work with you and find a dog that is exactly what you are looking for.

Yes, some do have issues but there are also plenty that don't.