I've seen this issue up close. (A former co-admin of mine had this very issue where they were pretty much a massive ball of negativity and issues. And it was frustrating to deal with them. There were times where I wasn't sure whether it was actually an issue or if this was attention seeking behaviour or what. Unfortunately, by the time I got so fed up with the situation and started to make changes I was just burned out completely. I ended up just shutting the site down entirely because I didn't want to be a part of that site anymore. I didn't want to put the effort in to revive it. And looking back, I'd been burned out on that site for a while. )
I understand that people have bad days. Even possible that they have a bad month where they just keep getting bombarded with problems. And having a rant now and again is fine. But for anything more than a one off occurrence? keep it off the forum/site. Dragging those persistent issues online brings nothing but pain and misery.
Others on the internet really can't help you. And over time people do get tired of hearing about other's problems (so much so it actually has a name...
Compassion Fatigue). So it's not that people are being callous purposefully, they just have no interest in hearing more misery, their empathy well is all tapped out.
So after a while hearing all this misery just causes people to distance themselves (member and staff alike). Like why talk to someone when all you hear is about how bad their life is or how crappy they feel? And it really does start to feel like attention seeking behaviour rather than a legitimate issue (even when they may actually have real issues).
Which, I think that, if someone is having that rough a time then they need to stop complaining on the internet and get professional help. Most admins are not trained psychologists or counsellors (and those that are likely don't want to deal with this in their off-time too). Most can't help deal with this stuff. And trying to help just puts admins in a bad place (no training at all and trying to help makes them responsible for that person in the event anything bad happens).