One thing I love about writing is that you can use backwards logic. You do not necessarily have to understand all the motivations when you begin a story. You can get sudden insights as you write and then you can go back and foreshadow/weave bits in.
Sometimes, the bits are already there, and others you need to offer a few more hints.
When an editor is looking at your work, she will be using logic forwards. In other words, does it make sense from the beginning?
But writers are not editors. Writers create and the process of creation can be messy.
Often the insight into why does not come until the last page of the first draft. Sometimes, it takes longer.
The simplest reason for this is if a writer knows all the twists, turns and motivations, the story is told and the writer may become bored. I love the fact that Ian Rankin has been known to write in capital letters things like THE GIRL FROM PAGE 20 COULD BE IMPORTANT. His books are tightly plotted but even he allows his daemon to play.
In other words, sometimes knowing too much can destroy creativity. If you understand the motivations, you might lose focus of the purpose. Objects can take on symbolism as the story progresses, rather than having their symbolism telegraphed.
In early drafts,everything is fluid. Lose that fluidity and your wordscan harden like concrete.
For me, I prefer I sketchy road map. Some authors like to have every blade of grass detailed. It just depends. It is another great thing about writing -- there is no correct way to write, and each manuscript throws up different problems.
In other news:
My cold is getting better -- hooray!