Certain posters enjoy being overtly negative, to lessen the enjoyment of others. I firmly believe that.
On the other hand, some people just like to call a spade a spade.
It really bothers me when people imply that somebody who is being critical or negative is a just a troll looking to incite controversy or conflict.
Sometimes that's the case, and when it is, it's usually blatantly obvious because they don't try very hard to think their reasoning through - they're just being jerks.
On the other hand, a lot of the time people who are being critical just aren't the type of people who feel like being blindly optimistic when things are falling apart before their eyes, and feel a desire to post their reasoning for what they are seeing going wrong. The main problem with that kind of poster is sometimes they tend to go overboard a bit, repeating themselves a lot.
But I think you have to acknowledge that the "woohoo come on guys let's be positive banner 18 no matter what!" kind of people have been, at times, just as hostile as those who are more in the "wow our team has looked really bad lately and i'm not confident things are going to get better!" crowd.
Some people comfort themselves when they are faced with the disappointment of their favorite team playing badly by rationalizing it and telling themselves that they could see it coming all along. They start to assume the worst, because that way they can't possibly be disappointed; at worst they are merely shown to be correct.
My dad does that, and it's very obnoxious. I've vowed not to watch New England sports games with him anymore because he repeats the same negative, pessimistic refrain about whatever team we're watching and always assumes the worst will happen. What's worst, I can tell he's doing it mostly just to get me going, because he knows I'm a big fan, and even though I try to be realistic, I also try to be as positive as possible.
So I can understand the desire to get away from people like that. But honestly I haven't seen nearly as many "negative Nancy's" as PLamb makes out. There are a few 'usual suspects' who say the same things over and over and seem to be trying to get people going somewhat, but overall I just think there's a lot of negativity because, unfortunately, there's a lot to be negative about right now.