I think it's a combination of realism and skepticism, and as a result are definitely far less likely to accept poor treatment or fall for lies. If you don't blithely trust 'nice people' you don't actually know (you know, like Tony Blair etc) then moaning about everything you don't like may actually stop it carrying on if enough people do it.
Of course you don't actually have to be miserable while being critical but the two do tend to go together as you can see the true misery and despair where others overlook it. That would make most people grumpy really I suppose.
"The data doesn't matter. We're not basing our recommendations on the data. We're basing them on the climate models."
Prof. Chris Folland, Hadley Centre for Climate Prediction and Research