If allowed to mouth you or other objects as early as from puppyhood, dogs will continue this behavior while into adulthood. Mouthing for a dog is a very gentle placement of your dog's mouth and teeth on your skin or an object. Dogs tend to mouth when they're excited or when they are greeting their owners or even guests. Having a dog mouth people can be scary if he is mouthing people who are not used to the behavior. It can also be quite messy, especially if you are dressed for work, for instance. Dogs explore the world with their mouths. As puppies, the mouthing behavior can become a little more intimidating because they will mouth or nibble a little harder as their teeth are coming in. Allowing your dog to continue to mouth your hands or your arms can be troublesome as they grow into larger dogs. If you can control it, stop this behavior early.
Training your dog to stop mouthing becomes a matter of giving him alternative methods of entertainment and discovering his world around him. Keeping your dog distracted and occupied during certain occasions of mouthing, such as when he greets you after a day away from you or when he's excited to go somewhere, can help to stop the behavior. You can provide him a method of entertainment for his mouth, so he has no choice but to give up the habit of using his mouth to show his excitement. If you haven't already, you could also begin to teach your dog commands to give him something to do in the form of an action rather than mouthing you during these occasions of greeting or playtime. Standing firm with your dog and stopping the behavior as it happens or ignoring him, so you are no longer feeding the behavior, will be crucial during this training. Starting early as a puppy helps, but you can train an older do to stop as well.
Be sure you have toys to offer your dog when he is mouthing. A favorite ball to put in his mouth might be helpful in getting him to stop. You will also need treats to offer him as rewards for a job well done while he is learning to stop mouthing. Substituting behaviors with toys and treats is acceptable during this training. Teach your dog basic commands before you start this training so you can use those commands to give him an alternate activity to do instead of mouthing.