It doesn't matter why people become negative at work. It doesn't matter it comes in the form of open denial of the truth of others, push back, yes-butting, opposition, cold silence, condescending questioning, bullying criticism. It's apt to shut others down, minimizing collaboration.
The most important response is getting more context and detail from them on the nature of their negativity. It's far more useful than defensiveness or counter-attack. It's getting to their assumptions which are typically behind all forms of creativity. It's a lot of patient persistence