When I got my GR Sam, he was afraid of everything (rescued at age 7 months). He got a fright at the local park and took off, luckily ran home. It took a long time for him to gain confidence. Eventually tho I realised he would subtely place himself between me and men he wasnt sure of. He was very sensitive to people and twice growled at guys I would have said looked a bit sus.
one night he was barking his scary bark, telling me someone was near, and when I went out, he had bailed up a young man in the garage.
I think I must be doing something to teach my dogs to do that, just wish I knew what!