Teething when yhtey are young, its painful just like for a human child so they chew or naw on something to make it better and help. Other times they are bored and find it amusing.
Depending on their age, they could be teething. When they are puppies and their teeth are just coming in, it hurts so they chew on things to help
Full grown dogs, because they are bored I think. Same as digging.
teething or they find it fun
hobby , old habits die hard