You know how your dog sometimes takes over your spot when you get up? There's a reason for that – and it’s pretty cute. The place where their human hangs out is a source of safety and security – so they are likely to go there when you leave the spot. They also see themselves as “protecting” you when they occupy the place where you once were.
Other reasons? If your dog has separation anxiety, being in the spot where you once were makes them feel secure. And for a dominant dog, especially with other dogs in the house, occupying your spot asserts their dominance. (Mental Floss)