Why is my dog so skittish?

My dog Cosmo is a 4 year old Parson Russell Terrier. I've had him since he was a puppy and he was never abused. He's gotten better, as he used to be scared of things such as trash bags and new furniture in the house. However, when someone is near him that he hasn't met, or may have met only once or twice, he barks and runs around them in circles. I try to call him to me, but he continues to bark at the person. He is not aggressive at all, but its like he can't hear me when he's barking at someone. He is almost always very obedient. I do not hit him when he misbehaves, so Im not sure why he is so skittish. It could just be his nature, but it is just very embarring when he refuses to come and will not stop barking. Help!