I like saladfingers81's analogy, and I'll take it a step further, because some people have asked why negative criticism attracts a stronger response than positive feedback.
Here's the analogy - you're in a pub, and you shout out "Cider is the best thing ever, and the people who like it are great!". A few people look around, maybe one or two raise a glass and cheer, everyone else ignores you. In the next pub, you do the same, but shout, "Cider is rubbish, and the people who like it are idiots!"
Which statement is most likely to provoke an angry response? Surely there's no surprise that you're more likely to get a rebuttal (possibly a violent one) from the people who drink it and take exception to your criticism, than from the people who don't but disagree with your cheering.
Negative feedback is very valid, but needs to framed carefully. There are many posters round here that do that on a regular basis, and I for one, love 'em.
Unless a flame war is what you're aiming for, of course.