Why does my Lab never growl or bark for intruders?

My Chocolate Lab never growls or barks. He is 1 1/2 years old. Even when the doorbell rings he won't bark to alert me for anything! Also he won't growl. I can rough house with him and he won't growl. I know he is a Lab but he is my first Lab ever! I have had Rottweilers and they always growled or barked. Should I be worried or is this normal behavior for a Labrador?