Disclaimer: no one in my life should take any offense to this. Please. Read item #5 on the list.
1. Because everyone will say “send me your novel! Send me your story! I’m dying to read it!” And you will say ok then, and send it along (blood,sweat,tears,etc) and, for the most part, no one will actually read it. Which wouldn’t be a big deal because your friends and family are busy and have the best intentions. Except said piece of writing is probably your heart and guts so it feels like a big deal and you will take everything personally and feel depressed.
2. A few people will read it. Some will say nice things, some will say helpful things, some will say things that completely contradict what you meant to do and show an absolute misunderstanding of who you wrote your characters to be, and exactly because this is your heart and guts on the page, you will take this super-personally and not be able to sleep.
3. Your book will change SO much between sale and publication. All of the things you try to explain to your readers may be irrelevant by the time it hits shelves.
4. Um. Nobody should ever have the chance to say “o, I read that. I don’t need to buy it."
5. Mostly, you should not show your writing to friends and family because if you have just sold your first book and are in any stage of pre-publication, you are probably highly unstable. And maybe slightly irrational. And re-read reasons one and two.