Reader/Writer/Reviewer

I find myself in a bit of a predicament. I started off as a reader. I love to read and read as much as possible. Then I became a writer. Published my first book in 2011. Now that I am a writer I realize how important reviews are and I try to provide authors with feedback on the books I read. This is something I never did prior to becoming an author. So here's the question what makes me qualified to review another authors book? I am just a reader and writer myself right? Why should my opinion matter? I mean I think it does but I am a little uncomfortable being completely honest about how I feel about other authors books. If I give a really good review does everyone assume we are friends? If I give a bad review will there be retaliation?
I recently finished a book that I knew that I couldn't give a good review. I knew the author from us being in some of the same facebook groups. Before posting the review I decided to reach out and let her know my thoughts and what my review was going to be. It turned out to be the best thing I ever did. So for now I will continue to read and I will post reviews and be honest with my opinions of what I read as a reader and not as an author.