I very much agree that it is a combination. Tasha has always been timid of people outside my husband and I. I got her from the shelter at 7 weeks old and socialized her extensively, took her to obedience school for the socialization aspect and so on. It just didn't matter. Once she got used to people she was ok with them, but she still barks and growls and stays well clear of visitors, especially men, even though she has never had a bad experience with a man. And the more loud/confident/aggressive the person is in personality, the less she trusts them. But I know she has never had an actual traumatic experience with people to cause the behavior, and she has been socialized as much as a pup could be.

She's MUCH more willing to approach and trust someone who has their own dog with them. Almost as if "hey if that dog is fine with them, maybe they are ok." Can hardly blame her for that though. I trust doggy people more too!