We're all beginners at one time.
Therefore, we should all be ever-willing to help each other.
However, there are good reasons why an author will say, "No," if you ask him or her to read your unpublished manuscript.
1.) There's no upside. Here's what I mean. One author I know said, "Every time I read someone's manuscript they get mad about my comments. They SAY they want me to be honest, but .... It's not worth it." Yep, honesty can hurt. And ruin even good friendships. Can you blame an author who'd rather have you just a teensy-bit miffed because she didn't read your work rather than have you MAJOR HONKED and actively bad mouthing her because she did?
2.) There isn't time. Writing is a full-time job. So is marketing. So squeezing in a critical read sure dries up any extra time. For writers who do two books a year, that's the difference between sanity and hanging on.
3.) It's not a professional relationship. Here's what happened to another author friend of mine. A man she met through mutual friends praised her books and asked if she'd read his manuscript. She did. She pointed out that 900 words is far too long for the genre he was writing. He got all huffy and told her she didn't understand what he was trying to do. Duh! In a professional relationship, you PAY someone for his/her opinion and you takes your lumps. But when we work for free, we equalize the relationship. It's much easier to dispute advice when you don't value the advisor.
4.) It's doubtful that the author has the time to work with you on corrections, so your very natural questions will have to go unanswered. And bad feelings will occur.
5.) You probably shouldn't be asking a professional author to read your work anyway. That's right. It's not the best way to get the indepth, critical analysis you need. Instead, you should be taking your work to a critique group or workshop. Or paying a book doctor or editor. Once you pay someone they work for you--anything you ask of the author is a favor.
6.) And here's the biggest reason my friends tell me they won't read another person's work: Lack of appreciation.
Here's an example: An author dropped everything to read a partial manuscript that a friend wanted to submit to a writing contest. He labored over the weekend making suggestions and edits. He sent it back to the unpubbed friend early so the friend could make the changes or discuss them. Then he waited and waited and waited. No answer. Two weeks passed. He called his friend who said, "Oh, yeah, well I forgot to thank you. I decided to send the work in before you finished because I just, well, I got anxious. But thanks. I appreciate it. Sorry I didn't get back to you."
When you take your work really seriously, it's very disappointing to spend time--time that you took from your life, your family and your work-in-progress--for someone who doesn't value your effort.
All this said, polishing your manuscript is the first step to publication. My agent tells me that she can tell if a work has merit 50 pages in. "Actually, I don't need to read all fifty pages. Most people have friends or a critique group help them with the first, oh, chapter. They go over and over that. But somewhere around page 30, THEIR writing comes through," she explained.
If your work isn't ready, it won't be published.
As good as YOU think your work is. It could probably stand to be improved. All of ours could!