Hella_Tellah
Explorer
Look, when you plan an encounter, you probably have a few ideas in mind about what the bad guys are going to do, how they might take advantage of the terrain or counter the PCs abilities or whatnot. Those ideas are hardly a "straightjacket" on how the encounter is going to unfold. An adventure or campaign arc story is a bit more involved, but the same principals apply.
There's nothing wrong with planning what the baddies will do in an encounter. But if you spend countless hours planning on having Hank the Barbarian carry the enchanted Halberd of Pabst Blueribbin to the sky kingdom of the winged elves, and Hank's player is more interested in bedding wenches and killing goblins in the forests of Daventry, there's a dilemma. Either the GM abandons his long prep work that assumed the players would act a certain way, or he finds ways to force Hank to carry the Halberd.
As a GM, it's easy to become emotionally attached to the plot I've developed, especially if it's awesome. Many GMs I've known have responded to player disinterest by trying to prove how awesome their plot is, pushing even harder against player apathy. This is why I suggest that people learn to wing it a little more and prepare a little less--the less you invest in a plot, the less you'll become attached to it when players reject it. The next best alternative is to prepare even more, and just have a ton of alternatives available. I haven't been able to do that since I was 20, but that can work, too.