"You have to write the story of my life! It would make a great book!"
Seriously. I don't write non-fiction, so it's not likely I'd write someone's life story. (Besides, I don't think I want to know you that well.)
"You have to write about my experiences at work/in the Army/in college/in Pick-A-State."
It's true that life is stranger than fiction, but a story requires a plot, with conflict, great characterization, and it's put together and twisted and shaped into a finished product. Stories about life experiences are biographies or non-fiction.
"I've always had this great idea for a book. I can give it to you and you can write it."
If you're so excited about it, you write it. We can't expect another person to be as excited about our ideas as we are. And -- I have plenty of ideas of my own. Time and marketability are the issue.