Any dog can be a "good dog." Any dog can be a "bad dog." Any dog can be sweet. Any dog can bite. It's mostly a matter of upbringing and training and environment. But different breeds, particularly pure breds, have decided behavioral tendencies and physical characteristics that you should consider when picking a pet. A pit bull can be sweet and gentle, but if it goes bad for whatever reason -- or for no apparent reason -- it's a greater danger than other breeds because of the extraordinary power of its bite. Chows can be wonderful pets, I'm sure, but there are no guarantees the it comes to dogs. Better to select a breed that is more likely to be a "good dog" and less likely to be a problem if it isn't.