It's very hard for people like that to not affect you. They're sort of energy vampires, in that they suck the fun out of a room, and they drain you of your positive outlook, even if you try to ignore them. A lot of people like that don't want to be "cheered up," and are in fact not "sad" or "depressed," but merely pessimistic people at their most base level. They WANT to see the downside to everything. I think once you determine that's the case with someone, you really only have two options: tell them that their attitude is really wearing on you and ask if they could find a way to not always point out the worst case scenario/worst parts of people/negative side of the situation, or simply tell them that their perspective on life makes you concerned for them, but you can't devote any more energy to them and leave.

If it's someone you have to be around (coworker, in-law, etc), I would say come up with some catchy come-backs that diffuse the situation and "jokingly" make it clear you don't want to hear any more. As in:

Them: "It's raining again. This whole area is so terrible to live in!"
You: "Beats the heck out of dying! Anyway, so about that report you owe me..."

Them: "I can't stand Kathy. She's so moody."
You: "Well, that makes a matching set! Maybe you guys should go on tour. So, want more popcorn?"

Well, my husband is one of those negative personalities, and I try to remain optimistic in spite of it. I feel compassion for people who think life is all gloom and doom, and meanwhile they're missing out on the joy life can bring. His negativity does bring me down once in a while, but only temporarily. I simply refuse to let other's emotions control mine. I just let sit in his dungeon of gloom until he snaps out of it.

Gradually he's learned to count his blessings, and when he gets in a negative mood he sometimes stops to realize that life could be a whole lot worse than it is. Living with him all these years has taught me infinite patience, and he does make me laugh so hard I cry sometimes, so it's been a long, strange trip, but well worth it.

eww negative people do suck as the saying goes. They seem to suck the life right out of a person which is an awful feeling. I deal with negative people by avoiding them, who needs to have a big downer?

When you are with someone who is complaining all the time, it puts you in a bad mood and soon everything is miserable. By avoiding the negative people I can remain somewhat upbeat in most cases.

Right now I am the negative type of person I despise so I am trying to stay away from others who are full of the holiday spirit. I have not holiday spirit and do not wish to bring others down.