"I want my first lines to be perfect. I've written 20k, and I've planned out the whole story in my mind. I know the tone I want to set and what my key themes will be. Will you help me?"
Um, no. Because a) I'm too lazy to waste time and effort on b) something that will invariably be re-fixed later on.
It's easy to think my response is mean until you stop and think about it. Personally, critiquing someone else's work, even just a handful of sentences, takes a lot of time and effort. I'm a high school English teacher, so I do A LOT of critiquing - only it's called marking, and I mostly get paid for it. I also have very little time to devote to writing of any kind right now, all of which means, if I'm going to critique something non-school-related, I want to think that it's the ABSOLUTELY BESTEREST BEST the person can do themselves before I have at it - otherwise it feels like a total waste of time, or else that THEY are being lazy.
So what's wrong with the above situation? Why couldn't these first sentences be as perfect as the author can possibly make them before they finish writing the book? Because of one thing:
Have you heard the phrase, "No plan survives first contact with the enemy"? Substitute 'first draft' for enemy (although, let's face it, the resemblance is striking :P) and you'll have the motto that every writer needs tattooed on... well, the back of their hand would be more practical that their forehead, because hands are visible while writing, whereas faces, not so much. Unless, I don't know, you write with a mirror next to you, or your webcam open on your computer ;)
Because here's the thing: For most people, most of the time, it doesn't matter how much you pre-plan a story, something will always crop up as you're drafting that will surprise you. That's an awesome thing, and it's the whole reason for me why first drafts are fun - but it also means that you'll never really know what your beginning needs to encompass until you're done with the rest of your book.
Sure, you might think you know the plot, because you've written it out in your head, and you might think you know the precise tone that you want to convey - but what if you have a brilliant spark of inspiration halfway through writing that will work with what you planned, but subtlely shift the emphasis so the story is even deeper, even brighter, even better?
There's a really good reason the pros don't do more that a light polish on drafted chapters before they type 'The End'. Actually, reasons. 1) It means you haven't wasted time polishing something that you won't actually end up using (same reason why you should always do your structural edits before your line edits), and 2) it means you can avoid being caught in the mire of 'If I Can Just Get This Perfect, Then I Can Move On'.
I'm not saying editing before 'The End' is always terrible. In fact, as I pull out my old half-written draft of The Project, I'm planning on doing some edits. There's a subplot that I really need to weave into the early chapters so I can figure out how it impacts the later chapters, so I'm rereading my draft, doing light polish-line edits and inserting the three or four (I hope!) scenes that I need to build my subplot - and then I'll write the last third of the draft.
But the difference here is that 1) I've had this book shelved for over a year, and 2) wow, look at that, my plan didn't survive first contact. The story changed. And now I need to figure out how to change my plan to match.
Sure, people are going to disagree with my assessment, and I know people happy to swear that their plan won't change, they promise, because they've been imagining the story for years and they know it so well. Each to their own. Just don't expect a lot of people to be clamouring to help you line edit when you haven't finished the book ;)
(Incidentally, this is part of the reason why agents and publishers aren't interested in incomplete manuscripts from unknowns - they have no guarantee whatsoever that what they read will actually make it to the final draft, or that it will even be RELEVANT.)