He's happy he got a new robot dog that does lots of cool stuff, and he thought he had met a kindred spirit in robot dog fans on the bridge. It's like talking about your Playstation 3 to someone who has a Playstaion 2. There is no way to have this discussion without acknowledging that the PS3 is better, but assuming the PS2 owner is also a video game fan, he will understand the context and not have his feelings hurt. And what if you show your friend how the PS3 can download pr0n (it's can't for all I know). It's a rude joke depending on who you are talking to. We have all played rude, immature jokes on people we knew and thought could handle it. The guy in this story is guilty of thinking he has made a stronger connection to the ex-soldier than he has. He does not move aside because he thinks the soldier is interested in what he has to say, and the soldier give him no reason not to (that I can recall).
Look, like I said, I know it's a fluffy little story and that I am over thinking it. I also know that my take on it is not what the author intended and that yours is. We've all been cornered by boors who wont shut up about stuff we are not interested in. They are often not evil people, but overly friendly and enthusiastic, and maybe lonely. But we don't punch them in the face or kick them in the nuts or kill them (or their dog, thank you eytanz). We say, "That's great, but I really have to be going."