Dogs bark for several reasons. Some of them include Warning you of danger, fear & trying to guard their resources. If you want your dog to bark in order to warn you of perceived danger, then you need to build a relationship with him. You and he have to become best friends. Since you have got an adult dog, it will take some time to build trust with you and love you. Then he will try to protect you against any perceived danger by barking.
Fear and guard barking are very different. If your dog is not doing those things, then its a good idea. That means your dog is a well socialized calm dog without unnecessary fears. Don't try to change that. A calm dog is a safe dog :)