Billy definitely has his dislikes, not unlike the dog above.
He doesn't care for cyclists, and he dislikes joggers. But most of all he doesn't like people who suddenly appear from behind him.
He's much better now with the cyclists and joggers, and he simply gives them a withering look, but if someone suddenly comes running-up from behind us, he'll probably try to snap at his or her heels. I keep him on a very short lead in such situations to avoid being sued.
All this only happens when he's on his lead. When running free he is much more tolerant. There is something about a lead that spells 'territory'. When he meets other dogs in the park they play together, no questions asked. But if he meets one on a lead, and he's on his lead too, then they always growl at each other. Again I imagine it's territorial, or maybe he's simply defending his owner.
Generally he's very well behaved, but there are certain behaviours that seem to be built-in to the canine mind, and there's little point trying to remove them. One simply needs to be aware.