My adopted dog is withdrawn, doesn't play or sniff. What should I do?

Every dog is different, and some dogs simply don't like to play with toys, bark, or sniff things when walking. David is also not a puppy, so he won't be as interested in playing as a puppy would be interested. You don't know his background either as a shelter animal, so it's possible that he came from a home where he was ignored and neglected, and that has caused him to be more withdrawn. It's a good idea that you keep the bedroom door closed to keep him from hiding under the bed. I would brush him daily, as that gives him positive interaction between you both. You can train him to sit, lay down, and stay, again forcing positive interaction. Give him treats and praise him when he comes to you for love and attention. Positively reinforce behaviors you want to him to have, which will make him want to do them more.