As an aspiring writer who enjoys telling stories, I know most authors write because they want to share part of themselves with other people. They're usually happy to talk about their craft and why they enjoy writing, but there's a number of things writers hear time after time that they never want to hear again.
1. “Wow, your writing is actually pretty good.”
Even if this is intended to be a compliment, it sounds like you're very surprised at the quality of their writing, and no writer wants to hear this. For one thing, it sounds like you had no confidence in their writing ability whatsoever. They'll probably think you were expecting to read complete garbage, and were pleasantly surprised by the averageness of their work instead.
2. If they let you read something, don’t critique their word choice.
That story, poem, novel, memoir, or whatever it is, is special to the writer. It's their baby. Chances are they put their whole heart and soul into that piece of literature, and unless they explicitly ask you for your thoughts, you shouldn't criticize their work, especially not their word choice. Until you actually sit down to write a poem or chapter after chapter of the same story, you have no idea how difficult it is to try to find the perfect word or sequence of words to describe what you envision in your mind.
3. Never ask them to stop writing to do something with you.
Don't get me wrong, naturally, this one has some exceptions and it's different for every writer, but as a general rule of thumb, writing time is sacred. It's incredibly difficult to get into the zone, and for those writers who wait for inspiration to strike, it takes even longer. However, once they've hit their stride, they have the ability to write page after page for hour after hour. Which means you shouldn't be interrupting them unless it's an emergency, because the second they lose focus on their work, they have to go through the whole process of zoning in again.
4. “Most writers don’t make very much money.”
Truly passionate writers aren't doing it for the money or fame; most writers are writing because they can't imagine not writing. They have a story, or a hundred, to tell and this is the best way to reach the largest audience. They don't need the reminder that their love of storytelling might not get them very far.
5. “You’ll have to get used to rejection letters.”
First of all, thanks for that vote of confidence. Second of all, you think they don't know that? If a writer is trying to get published, chances are they are already aware of the trials and tribulations they'll have to go through. J.K. Rowling's Harry Potter was rejected by 12 publishers, and that's one of the most successful book series of all time. So writers know they're going to get rejected, they don't need anyone reminding them of this or implying that their work deserves it.
6. “Isn’t writing a dying art?”
Writing actually isn't a dying art. It's true that fewer people write letters to mail to one another nowadays, but other than that writing is still very much alive. Writing is active in many aspects of everyday communication, and more so than that, as long as people are telling each other stories, writing will be around. Not to mention, even if it is a false statement, no one wants to hear that the thing they love doing most in the world may one day be obsolete.
7. “Can I be a character in your novel?”
If a writer puts someone they know in a book or bases a character on them, it's because that person has made a tremendous impact on their life. Which means if you deserve to be in the book, as either a protagonist or an antagonist, you're probably already in it.
8. "When is your book going to be finished?"
We appreciate you're anxious to read it, but it takes time to craft a masterpiece, and that's assuming the writer spends all their time working on just one idea instead of abandoning the idea several pages in and beginning work on something else. It's impossible to know exactly when they'll be finished, and the process takes that much longer when writing isn't their day job or primary focus.
9. "Oh, you like to write? That’s a fun hobby."
I can almost guarantee that everybody who has ever said they want to be a published writer, has heard this response, "Oh, you like to write then? That's a fun hobby." Or some variation of it. For these people, writing is a lifestyle and it shouldn't be dismissed as a mere hobby.
10. "Anyone can write."
First of all, no. While it is true that a great deal of the population is capable of physically writing, there's still a vast majority of those people who can't write grammatically correct or with proper spelling. Even fewer can write well. When writers say they write, they don't mean stringing sentences together. They mean creating whole worlds and characters strictly from their imagination.