Giving off bad vibes

Why are there people who, when presented with a great opportunity, find someway to shoot it down? Or you spend the whole time complaining, and saying how YOU would go about it, what YOU would do, what YOU would change, and how things would be so much better if YOU were the one running the show. You're always looking for the mistakes in everything. Why are you so pessimistic? You're not only ruining your day, you're also ruining mine and those around you.

Why? Why? WHY?

What kind of pyschological damage have you sustained to refuse to see the good?

I hate people like that. They suck out all the happiness in a room until it's dark and dank and smells weird.

What do you do with people afflicted with "defeatist syndrome"? Can we all just chuck them on to a deserted island somewhere and forget about them?