As a GM, it's never a good idea to leave your players alone for any length of time and have them continue player interaction.  You might wind up coming back to a game of Call of Cthulu. 

Player interactions are fun.  There's almost always a time where one player can exact revenge against another.

Sometimes the role of GM is more like being a conductor than anything else. Keeping things on the rails is hard, hard work.  Threats of bombs going off is sometimes required to keep the train rolling.

Sometimes the most frustrating thing about being a GM is not knowing if your players are making a joke, or if they really are clueless.

 The main function as a GM is to describe the world to the players interacting with it.  This includes describing things happening to the players, including any pain associated with that action.  Sometimes GMs take great pleasure in this aspect of the job…

Make sure that when inserting foreign objects into player characters that you have enough henchmen to hold them down.  Nothing is more frustrating than when a player rolls a successful dodge vs. insertion.

Wow, that sounds totally inappropriate. 

Players sometimes get a god complex where they think they are untouchable.  Making your players powerless for a short time and having your way with them drags them back to the reality that their players live at the GM’s whim, and you don’t mess with the GM.

 As a GM, in order to keep the game moving forward, you sometimes have to use executive power and override player freedom.  This will be met with resistance, and it’s a delicate balance to maintain.  The power can be used too much or abused, and that usually spells death for the game.  Throw the player a bone and give them the illusion of a savings throw.

And to translate the last frame: "Savings roll, Bitch!"

Some players don’t take well to threats or blackmail.  Even when you hold all the cards, players will sometimes still pull at the leash.  

 Sometimes GMs are the embodiment of evil.  We just tell ourselves it’s for the good of the game, to make it more believable, if we have trouble getting to sleep at night.