First off, it's probably a holdover from my childhood: "If you can't say something nice...." And, unfortunately, over the course of my life, there are probably a lot more instances when it was better for me to keep quiet than to say what I thought of a particular book. Or at least, I thought it was better.
Secondly, there's a fine line between saying you like or don't like, love or hate, a particular book and writing a review. A review means you have to look beyond your personal likes or dislikes of a particular work. It's like looking at a painting of, say, Van Gogh's "Starry Night" and saying you love/hate it because it has a lot of blue in it. And blue is your most/least favorite color. You have to go deeper than that.
When offering or asked to review a book, you have to be honest with yourself about how objective you can be. If something as trivial as the fact that the protagonist's name is the same as your spouse's ex can make you hate the story, gracefully decline the request to review. You can't make this personal. Also, if you're just not a huge fan of sci fi/fantasy, political thrillers, westerns, romances, etc. then don't offer to review a book you normally wouldn't read. You have to be willing to really get into the story in order to make your review worth reading.
And then there's the hardest part of all... what if you really, REALLY didn't like the story or if the writer didn't do a very good job of writing the story? What if, after struggling through to the end, you can't find much good to say other than the writer is a good friend of yours or you've liked other work but this one just doesn't do it for you?
This is where writing a review can be tricky, especially if you're having it published in a public forum, especially if it's a product review. A good review can be beneficial to a writer, but primarily it has to be beneficial to the potential reader. If you write a glowing review of a book that is badly written, thinly plotted, or outright boring, you're not only not doing any favors to the reader, you're not helping the writer, either (assuming, of course, that the writer is a professional who WANTS to improve, but that's another topic for another blog post... one titled "Don't Be That Guy".) People don't like to be tricked out of their money and an undeserved glowing review may inspire someone to plunk down their money on a book they are not going to enjoy. And they're not going to like you, the reviewer, very much, either.
A good review should be more than just a simple, "This book was amazing and everyone should read it!" cheer. Why did you think it was amazing? Did it make you cry, laugh out loud, really think, keep you up all night? Why should everyone (define "everyone") read it? Because it makes sense of suffering, it lifts the spirit, it draws a picture in a way that has never before been seen? There's no need to give an outline of the entire book including the climax and denouement (please, don't!), but tell us what the story is about, who the main characters are, and what you liked best about the story (the dialogue, the humor, the descriptions, etc.)
And lest you think a good review has to be four or five stars and unrelievedly praise the book, think again. A good review can also point out flaws and problems with the book... but it doesn't have to be mean-spirited and cruel. You don't have to like the book to write a good review (at least from the reader's point-of-view) but it's not necessary to totally trash the book or the author. And a good author will take a good hard look at a "bad" review and learn something from it (it might even be that the reviewer doesn't know what he or she is talking about, in which case it's best to just laugh and go on about the business of writing another book.)
In the last few months I've gotten pretty good at writing reviews and I've been asked to write reviews for fellow authors (even got to write my first back cover blurb, booyah!) I look at it like this: I've been given a great responsibility, not just to the author, but to the reader as well. And as a reader, long before I became a writer, I appreciate the honesty of a reviewer who takes the time to tell me what it is about a book that I will love. Or not.
Plus, I get to read a lot of books and not feel guilty if dinner is late. After all, it IS work!
Oh, the trials and tribulations of reviewing books....