So, now I'm obligated to shake everyone's hand...?

So, I was talking to someone I know at the gym tonight... just a casual conversation and I was mostly holding my own and then someone the other person knew came up and started to talk to him. The person I was talking to introduced me and I smiled (I'm sure) and they went on talking. After that other (new) person walked away, the guy I was talking to asked me, "why didn't you shake his hand?" I didn't know what to say.

It never occurred to me to shake his hand. I mean, I know all about shaking hands, but I didn't even think it was a necessary task in this situation. I mean, when did this become an obligation in every situation? It was kind of annoying that I got asked this question.

So, this is one more thing to keep track of, I guess.