A few years ago I sat on a panel about running tabletop RPGs. I got asked what I do I do to ensure that players at my table are entertained. I said "I am Crom. I don't care." This got everyone's attention. "I don't care because that's not my problem. Players are solely responsible for their entertainment. All I do is run the game."Yeah. It's not the job of the GM to dance around as the cloun to arbitrarily "entertain" the subjective whims of the players.
Certainly, we all must give something to the game to get stuff back out of it, and the GM has a lot of power to determine the state of the "world" and how it reacts to the players, but ultimately, as a GM, I cannot ensure the player is entertained any more than I can force him to be happy when sad, or vice versa.
It also reminded me of one thing from one of the few TED talks worth a damn - Mike Rowe on dirty jobs - where he is talking about crab boats in stormy weather. He talks to the Captain, concerned about safety, and gets back the reply that the safety of the crew is ultimately their responsibility. Sure, he can train them, etc., but in the end they have to ensure their own safety, and his job was to get the boat home safely with a good catch, to make them a living.