[Roa_Aoife]

Some things are indeed meant to be in the books. However, the fortelling of the future is not dictating fate, as is the common misconception. It is, rather, what will happen if things keep going the way they are now. Cuase and effect. The fact is that certain actions are predictable, perhaps more so to the Ainur and the Elves, who are indeed bound by fate and the will of Iluvatar, and are old enough to pick up the general patterns in history. Men have free will and the ability to change the course of events, but often fail to realize this, and may believe themselves to be trapped. As for the reference to Gandalf and Durin, Gandalf was presenting a set of choices. Each choice has a set of consequences. The effect of a single hobbit was realized by a particularly wise (understaement) individual.
Another thing to be pointed out is that in really old world myths and legends, the main antagonist is beset by a list of taboos that will inevetively be broken or a curse that cannot be escaped. Since Tolkien's goal was to give England it's own mythology, writing in this style seems the logical thing to do.