1. Being around lots of flesh-and-blood people drains you. Of course, this does not apply to the characters in a book.
2. You enjoy solitude, especially when reading a good book.
3. You don't have a large group of close friends. I mean, not many are willing to play second fiddle to imaginary people.
4. You are often quiet and reserved. This is especially true of introverts who write because they're gathering information for their books.
5. You're not fond of hectic environments. It distracts you from more important things, like reading.
6. You tend to be self-aware. I mean, if experiencing the lives of all those characters didn't have a positive effect, it would be surprising.
7. You are a people-watcher. How else are you going to gather ideas for writing your own books?
8. You are drawn to jobs that allow independence, like writing. How else are you going to make time to read?