An elderly neighbour of ours, recently deceased, used to be very upset with us for keeping a large dog (Daisy's medium, actually). I guess he wasn't very mentally stable, because besides yelling at us how wrong it is to keep such a vicious dog (she never did anything but bark), he would also frequently bark back at her - quite impressively.
We were a little concerned at the time that he might do something about it (from complaining to our landlord to trying to hurt Daisy), and somewhat relieved when he moved out (apparently to a home for the elderly).
I just now realised that he may have sometime witnessed a dog doing something very vicious, and that it would explain his extreme reaction to being barked at. I suddenly don't dislike him anymore.