Why is my 3 year old dog acting as if he is teething again?

He is a tibetan mastiff/eurasier mix and has calmed down a mive amount since I first got him in 06. he stopped trying to teeth on me and others around 1.5 years old, but still liked to chew on peices of wood. He recently began trying to nibble on me and my family again whenever we play with him. Or if we tell him to get down when he tries to jump up, which he has learned not to jump on people, but instead jumps in front of them without touching. Any ideas?