Why are writers so critical of each other?

I recently got into a forum debate about a bestselling book that I liked (but, apparently, quite a few people didn't). In defending the book, I inadvertently sparked a lot more criticism of the book, much of which seemed awfully harsh.

Last night, while reading The Last of Her Kind, by Sigrid Nunez, I ran across a mention of a quote that perfectly described what I felt about all the criticism I was reading about the bestseller I'd stood up for:

"One cannot review a bad book without showing off."
--W. H. Auden

Particularly coming from a writer, criticism of another writer's work just comes across as self-righteous, as if there is always an unspoken I could have done it better. Not only that, though; often the criticism seems unreasonably harsh. Writers tend to pick on things that "normal" people would never consider a serious enough flaw to ruin a book. It reminds me of a theory in social psychology, which observes that people are most likely to judge others that are most like them. For instance, women tend to blame a rape victim for bringing it on herself, presumably because all women are vulnerable to rape, and therefore prefer to blame the victim and deny that it could happen to them just as easily, rather than admit to the vulnerability. I think this theory could also be applied to how writers react to each others' work: when writers see "just another writer" become a bestseller, they automatically find reasons to criticize, indirectly making the case that they are more worthy of bestsellerdom by denying that the bestselling writer is any good.

Now that I've stated why I think writers are so likely to turn on each other, I have to say that I don't care why they do it, I think it's absolute crap. All of my fellow writers out there - particularly the newer ones, who will remember their own periods of vulnerability better than the established, successful writers - don't you remember how much senseless criticism can hurt? I'm sure all of us have bad memories of someone making some ridiculous, hurtful, and not at all constructive comment about our work. Rather than turning that same fire on another writer, we writers ought to stand by one another, as we all know and understand what it is like to be injured by criticism.

I have one other quote that I want to leave you with, which Nunez also led me to:

"Three things in human life are important. The first is to be kind. The second is to be kind. And the third is to be kind."
--Henry James

Please, my fellow writers, be kind to one another; support one another.