It is a natural behaviour for dogs to be a bit possessive of their stuff. If they were still living in the wild, they would need to protect their main resource, namely food, in order to survive. Resource guarding can be defined as behaviours that discourage others from coming near or taking the item that is being guarded. Resource guarding is an anxiety/stress behaviour. Living with humans, dogs resource guard a variety of things.
Here is a step-by-step video on what to do:
See the June 2019 issue of Animaltalk magazine for more information.