Negativity costs
I'm not a Pollyanna. I don't see everything as being made of unicorns and rainbows, nor do I walk around smiling all the time. (I've been told my normal facial expression is somewhere between grim and "I'm going to kill you now," but I can't vouch for that because, well, the expression is on my face.) I do see the dark side of many things, and I certainly walk through life with more than a bit of paranoia.
All of which may make this statement surprise some folks: I'm really tired of dealing with negativity.
Constantly seeing the negative costs not only the viewer but those around him. The viewer pays by having a shittier than necessary life--and isn't there enough shit in all our lives?--and by missing so much of the good things going on all around and even to him. Those around negative people pay by having to help cheer up those they care for and by having their own positive attitudes constantly battered.
Negative attitudes also make work harder. Yes, considering all the available data is vital, and yes, some of that data will be negative. Constantly finding fault with every option everyone offers and yet coming up with no alternatives, however, isn't being clever; it's being destructive.
We all get down. We all wish parts of our lives were different. We all want things we do not get. If, though, we respond to these and other life pressures by constantly being negative, then we should consider getting a little help. The costs to negative people and to those who are around them are too high.