quoting Gygax

By: Unknown on 15:37

Read how and why the system is as it is, follow the parameters, and then cut portions as needed to maintain excitement. [emphasis mine] For example, the rules call for wandering monsters, but these can be not only irritating -- if not deadly -- but the appearance of such can actually spoil a game by interfering with an orderly expedition.

...

Know the game systems, and you will know how and when to take upon yourself the ultimate power. To become the final arbiter, rather than interpreter of the rules, can be a difficult and demanding task, and it cannot be undertaken lightly, for your players expect to play this game, not one you made up on the spot. By the same token, they are playing the game the way you, their DM, imagines and creates it.

...

As the DM, you have to prove in every game that you are still the best. This book is dedicated to helping to assure that you are.

All the above come from the introduction to the 1st edition Dungeon Masters Guide. In my original copy (I'm on my second copy, having worn out the first one) that last passage is outlined in pencil. I did that back around 1983 or so.