At my last job I had been there about a year and a half and there had been some discussions about expansion which included hints of a possible promotion for me. Then one day I came in to work and my boss was gone (no reason to go into details). In the month it took district to find a new boss, I in many ways kept the place running, and how was I rewarded? With a boss who wasn't interested in a word I had to say. To use a Star Trek metaphor, I went from being Commander Riker (2nd in command) to a red-shirt ensign whose always the first to get phaser-blasted on an away mission.
It was demoralizing and frustrating and to top it off the new boss had the brain power of a slug. At any rate I became extremely negative. Not to everyone, but to management I did nothing but complain and I'm sure they found me terribly annoying. Looking back I can see the futility in it. Even if I was theoretically right in my complaints, negativity rarely gets one anywhere.
The point of all of this is to say that while a few people are possible eternally negative, I suspect most people are simply in a tough arc of their lives, and oftentimes as outsiders, we don't have the full story to understand the place they are in. In most cases it's probably worthwhile to try and discover that full story. Most of us, I suspect, are much better off with a friend than a critic (though a good friend sometimes needs to be critical, they do so as a friend with the full picture).