It seems that when you tell some people about some good/exciting news, they feel the need to play the devil's advocate. Now I can understand the desire to see both sides of a situation, but some people take this to the extreme. Rather than just letting you bask in the glory of your news, they figure you need a little puncturing to return you firmly to Earth. In order to achieve this charming goal, they riffle through their encyclopedic knowledge of bad stuff that happened to somebody one time, and trot out the litany of unhappy things they once heard about. I think of these as Eeyore people.
In many cases, it can be almost any type of news that causes the Eeyore reaction. You could be telling about how you just ate a particularly delicious sandwich, and one of these people would feel the need to tell you about a really terrible one they once had. By the end of five minutes, they would have moved on to that story they once heard on the news about a sandwich that was so bad it actually melted off a woman's face (or some other terrible end).
In general, the bigger the news, the bigger the Eeyore reaction. College choices, getting a new apartment or buying a house, buying a car, getting a new job, getting engaged, getting married, and having babies are all huge triggers for Eeyores. There really does not seem to be any way to shut down the Eeyore (at least none that I've discovered) but at least one can try to limit the length of their tirade.
The one good thing that's come out of my encounters with Eeyores is an improvement in my own negativity. It may seem tempting sometimes to talk launch into a story about that wedding you once went to where the bride caught on fire/exposed herself/died of the plague, but it really is better just to hold off. If nothing else, save those stories for the car ride home. Above all, don’t let those Eeyore people get you down.