Why is my dog so protective?

She's still a puppy, a mix of boxer beagle and bullmastiff. She's a sweet girl and I love her, but how can I stop this? Whenever someone knocks at the door she stands right by it, growls, and barks her head off. Same thing with when she's in the car or sees another dog. Sometimes she won't even let my other dog come near me. Do you think getting her neutered will calm it down?