Prince my Lhasa has always been leery of strangers. He warms up to new people fairly quickly, but he is very growly when they first meet him. He's been like this since he was about 1 and a half. We chalked it up to fear and treated it as such, letting him meet new company only after everyone is settled, dragging a leash, the whole deal.
In over the 10 years that our neighbors have lived near us, he's never had a problem with them. He isn't fond of the kids, but loves the mom and tolerated the dad. His recall outdoors was never an issue until recently.
Within the last 4 or 5 months, he has suddenly become aggressive when it comes to my male neighbor. He's still fine with the woman and two kids, but when "A" is outside, he ignores every command given to him and runs full tilt snarling toward the man. "A" is the only person he does this with.
As background, "A" has never done anything to him. When Prince is outside, he is watched and never left alone where anyone could mess with him. He has been to the vet and there are no medical issues that could be causing this sudden aggressiveness.
At this point, we're resigned to taking him out on a leash when "A" is home and only letting him exercise in our yard when he is at work or at night when "A" is in the house. We'd like to be able to resolve this issue, but if not, we will deal.
My question is this: what could be causing this sudden aggression with just one person? What can we do about it? Should I be worried that this could turn into a problem with more than one person?