Because you have entered the Unknown. I am assuming that what your talking about is D and D, Vampire the Masq or Pathfinder. Running RPG's is both easy and hard. To be honest being a DM is a lot like herding cats. But, it is something you know. You know when you set up that underground maze there are only so many different creatures you can put in there without killing your party off one by one. You know what treasure is going to be around the corner because even if you came up with it, it's based upon something you have done before.

What you dont know is all of that stuff right now. There are no paramiters. There are no limits. Killing off someone might help the story rather than hurt it like it would with your party. This is completely virgin territory. So embrace it. Accept that you might not know what is around every corner or where every dead end is located and write your way through it. That is where your Muse comes in handy. S/he knows the way so let them be the scout that guides you through the dark. Darkvision will help. (e:smile)