Doctors just take everything a tad too seriously.

People don't take it well when you laugh at their stupid question/comment assuming it was a joke, when they were actually serious. Such people also tend to be more than a little unhinged, so it blows up into this huge thing. Most doctors make that mistake exactly once, then everything is deadpan after that.

That said, I work in the ER, so I try to defuse the tension & drama people bring in with humor. You really have to read your audience and start out very innocent. Conscious sedations are great though. I can unleash my irreverent gallows humor with an audience that's well past the "two drink minimum", and if it falls flat then they won't remember a damn thing when they wake up. When they stop laughing at my jokes, I yank their shoulder back into socket.