It seems natural. You write a few pages, or chapters, or a whole book, and you want a second opinion. So you hand your newborn literary baby to the person closest to you and ask him or her for feedback.
I mean, they're right there, right? So why not use family, friends and neighbors as sounding boards?
I'll tell you why. Unless they're the worst family, friends and neighbors in the world, they really want to like your book. Because they like you. And, perhaps more importantly, they want you to still like them when they're done reading your manuscript.
Aside from the unfair pressure this involuntary editing role creates, consider this more pertinent drawback: what you write might not at all be what these kindhearted folks enjoy reading. In the case of non-fiction, the topic may be incredibly boring to them. Or offensive. Or ridiculously unimportant. It could be a book they would never, ever choose to read, were it not for you thrusting the manuscript in their hands. So how in the world can they be expected to critique it?
But let's say you've already given the book to a work friend and he's finished reading it. Now what? Are you expecting sentence-by-sentence analysis? Objective criticism of plot transitions? A conversational deep dive into how your approach to Victorian vampire romance differs from the supernatural shenanigans of other early 20th century cultures? (Clearly that last one will NEVER happen, and if it does, you must tell me where you work and what else this friend does in his spare time.) The point is - and I'd be willing to bet on this - the response you receive will be one of the following: 1) I really liked it; 2) I wouldn't change a thing; or, 3) You're a really good writer. None of these are necessarily true, they're just an expedient way for your indentured reader to end the ordeal.
Even more disappointing, none of these responses give you what you're craving. What you really want is someone to tell you how to make your book better. No one who loves you will tear out your heart - or tear up your pages - and demand that you start all over again. And THAT'S who you need as your proofreader.
Want to hear how great you look since losing 20 pounds? Your husband's the guy to ask.
Want to know if two flashbacks and a flash forward are too much for one chapter? Ask a literary professional whose job it is to give you an honest answer.
Unless that literary professional is your spouse. If so, hire someone else.