As contradictory as it sounds there are good negative and bad negative people out there in the world and the ones that you want to hang around with are the positive-negative people as they will make the best walls to bounce your ideas off and critique your work honestly. If they don't like something then they'll speak up and let the chips fall where they may.
At the other end of the scale are the negative-negative people and these are the ones that you want to stay away from, A double negative is the kind of person who, upon hearing of your dreams of becoming a published author and knocking King and Rowling out of the best sellers list forever, will attempt to try and burst the bubble.
It may begin with a series of quick jabs to the ego "But you can't write a story, you can't even tell a joke properly" "What's the point? nobody reads anymore" "Doesn't sound like anything I would ever look at, though I suppose it's all right for some"
But they won't stop until you throw away your work and never again take up the pen for anything more exciting than a shopping list, a boring shopping list. This is the only time that they'll be happy because a double negative is an emotional vampire who feeds on anguish. 
Once upon a time I used to work with a double negative, for the purposes of this story I'll call her Impedimenta.
If someone took time off she disaproved of it because it meant more work for everyone else, but especially her.
When another worker went on a course she spoke badly about it because it wasn't going to teach them anything important. When she was informed that it was a first aid course she replied "So? he'll never need it"
My turn came when I was drafted into teaching a computer course that was very far from work and the two jobs would overlap. Clearly this meant that I would be late for this, my real job. Besides, what were those people doing on a course? Why didn't they go out and get jobs?"
It wasn't an ego, why aren't they talking about me?, thing at all. She just didn't seem to want anyone to succeed at anything, ever. Eventually the rest of us stopped talking to her which is exactly what she didn't want since it meant that she had nothing to disaprove of and that's when she sent her flying monkeys, as we called her minions, out to gather intelligence. 
Using them in writing:
I know I've just said stay away from the more extreme negative person if you can but unless you spend some time around these people then your characters are going to be living in a sacherine sweet world with a big purple dinosaur for company.
Life dictates that you must rub shoulders with these people and since we're writers it is an opportunity to take notes of the human condition and use them in your work. 
Perhaps your own Impedimenta becomes a major antogonist for your characters, who set out on their quest in order to prove her wrong. Or it may be that she is relegated to the chorus and her voice is lost in the crowd that cheers for the rightful king.
Take it all in but, and this is the important bit, remember to get it all out as well. Remember that their ultimate goal is to turn you into a double negative so be wary of the amount of time you spend in their presence and then go to a lake and feed the ducks, because it's impossible not to smile when your feeding ducks. 
1: with some nice metaphor for afters.
2: dun Dun DUN!
3: possibly my most pretentious sentence ever.
4: unless you get chased by the swan mafia who are desperate to get whats in the bag.