Any stranger who reaches out to a dog to give it a "friendly" pat on the head before introducing themselves to the dog (Holding out a hand for the dog to sniff, asking the owner, etc..) deserves what they get. That having been said.....

One of my brother's dogs had a problem with me for years, it seems I smelled like or resembled someone who abused the pup. If my brother was with me everything was fine, but she barked and growled at me anytime I walked into the house. It may just be something you have to watch for and deal with on a situational basis. Lady nipped a few times at kids but over time she's gotten more secure and it has almost disappeared.