Object-guarding starts during puppyhood . Owners may fail to notice their adolescent dog becoming increasingly possessive and protective. Some may actually encourage their puppy's protective displays, thinking they are cute.
It is natural for dogs to protect their possessions. In the wild, a wolf would hardly pop next door to borrow a cup of bones. Domestic dogs quickly learn that once something is gone, it is gone. So it is not surprising to find dogs trying to keep their possessions away from people.
Bitches are more likely to guard objects than male dogs. In a domestic pack, it is fairly common to see a very low-ranking bitch successfully defend her bone from a relatively high-ranking male dog. In fact, the Bitch's First Amendment to Male Hierarchical Law is "I have it, and you don't!"