I had a friend like that. I tried several conversational tactics to move her off of her negativity, including asking point blank what was going on for her that was good or joyous. She would sometimes mumble something before going back off in her negativity very soon. I finally decided that her negativity was too heavy a burden for me to carry, and I haven't talked to her in most of a year. I like it that way. I can manufacture plenty of negativity on my own. I need more positivity around me, and I choose my company accordingly.

Dunno if that helps, but that is my story.