In his 2000 craft book On Writing, Stephen King says,
The first draft — the All-Story Draft — should be written with no help (or interference) from anyone else. There may come a point when you want to show what you’re doing to a close friend. My best advice is to resist this impulse.
As you write the first draft of your novel, there are going to be moments when you want to show it to someone.
It’s not going to happen every day, trust me. There will be days where everything you put down on the page is total shit. There will be days where you struggle just to get the words down at all. These are not going to be the days you want to share anything.
But there will be occasional moments when the writing is going really, really well. When the last two scenes you wrote are freakin’ awesome, and you’re walking away from the laptop smiling and cheering for your talented self.
You’ll think to yourself, I have to show this to someone, I have to share this right now!
Yes, that impulse will come here and there when you’re working on the first draft of your novel. Novels take forever to draft, after all. Sometimes a month if you’re really fast, but often two months or longer. It can be hard to write your fiction week after week and not show at least a page or two to somebody.
But I agree with Stephen King — you really need to resist this impulse.
Patience is such a key part of success in writing. Patience, and hard work, will make you stand out from the crowd.
And being patient about showing people your writing will be the best decision in the long run.
Because no matter how amazing you think the pages you drafted yesterday are, no matter how fantastic chapter 15 is in every way, it’s still important that you not show it to anyone yet.
Why?
One, the scene is probably not as great as you think it is.
It’s probably filled with typos and misspellings and confusing paragraphs and awkward sentences that you won’t ever notice until the second draft, maybe even the third. Character motivations may not be super clear either. Why share your work until it’s ready?
Two, what’s to gain from sharing pages of your work-in-progress anyway?
Let’s say that your chapter 1 is amazing. No typos, no misspellings. It flows, it’s great, it works. You share that chapter with a friend or family member or writer you admire, and that person says, fantastic! I love this! I want to read more! Then what? Do you show them everything? What if they love the first few chapters, to the point where you become too intimidated and self-conscious to write the rest of the book?
Three, what’s most important, yes, is that you finish the first draft.
The whole point of the first draft is to get the story down as best you can… and finish it. I’ve met so many talented writers over the years who write beautiful prose and wonderful stories and three-dimensional characters… but never finish a goddamn thing. There’s so much more to do with a mediocre first draft of a novel than an absolutely sublime novel that never gets finished. Your goal in writing a novel is not to share pages you like with others as you’re going along but to keep that first draft to yourself and do the best you can with it until you reach THE END.
And when you finally finish that first draft, STILL resist the impulse to share your work.
It’s done, it’s finally done after months of work, so you’re naturally going to want to hand the manuscript over to somebody. A partner, a best friend.
Don’t do it. I’m telling you… do not do it!
Instead, put that first draft in the drawer for at least a month, if not six weeks or longer, and let it rest. Let it just sit there. And don’t let anyone read a word of it.
Then after that time has passed, take the manuscript out of the drawer and start your second draft. I guarantee that you will be so thankful you never showed anyone that first draft. Because you will see hundreds of mistakes in your manuscript. You will fix most of them. You will make that book better.
Did you finish the second draft? Good. Awesome.
Now put it in the drawer again, and then do a third draft of your novel in the weeks to come.
When you finish your third draft, then, and only then, should you show your novel to anyone.
I know it hurts. I know it sucks. I know this process takes forever.
But if you want to be a successful writer, I’m telling you, you need to wait it out.
Show your work to others when it’s 80% there, 90% there, not when it’s 30% or 40%. You want people to look at your work when the story is exciting and believable, and your characters makes sense, and the prose is as polished as you can possibly make it.
You want people to help make your good manuscript great. You don’t want them to look at your poor manuscript and then never want to read another word you put down again.
Enjoy the process. Write the novel for you at first. Don’t rush. Don’t feel like you need to show it to anyone, not for awhile.
The time will come, I promise. You just need to be patient.
brianrowebooks.com 
Totally agree. Prematurely showing work just pollutes the creative process for me.