Some writers love the revision/rewrite process. Some loathe it. Some grow too big for their britches and skip it altogether, then shit all over the Amazon.com comment pages when called out on it. But most will say that rewriting is a necessity. No matter how brilliant your first draft may feel, how effortlessly it poured from your fingers as if the Muse herself were manipulating your hands upon the keyboard, there will be something wrong with it you're just not seeing.
One of the mistakes I've found in my own writing happens when I know just where the story's supposed to go. I know Character A needs to get to Location B by the end of Chapter Seven – it's a given. So while I know this is to happen, and the character agrees to do it, the rush to get there may leave a reader going 'wait, but WHY?'
Then comes the painful part. You need to grab a big pair of imaginary scissors and start cutting holes in your precious tapestry. Then you have to make patches for those holes that address the things that you missed the first time, and stitch it all back together again without leaving any clues to the process. It scares the shit out of me each time I need to do it.
But I still do it, because without motivation, action has little purpose.
I would wager that the vast majority of people make decisions more on emotion than logic (otherwise we'd be Vulcans – and we're simply not that awesome). Even when we try to be logical, the limbic brain will sneak in, masquerading as logic, and fuck everything up. Even when your character has all their ducks in a row, some of those ducks should – obviously or subversively – represent what the character wants or fears.
And then you need to yank the proverbial rug from under them. The answer to 'why did she do that?' could be simply 'she was fucking scared for her life', but show us why. What threatens her life so badly that her reaction was so life-altering? Don't be afraid of getting messy. Kill off a beloved friend, or have the protagonist barely escape from an attack with her life. The only reason the reader should be confident in the protagonist's survival is that protagonists historically have a higher chance of surviving to at least the end of the book. But don't be afraid to play with that assumption as well.
All right, off I go now to torture some more people. Fictional people. Yes, totally not-real people. Ahem.