In general I think lack of empathy is the main problem, though I think even well-meaning people can make others feel guilty about having normal negative emotions, e.g. “You’ll get through it”. IMO the worst may be the one-uppers, the guy who says “That’s nothing. I knew this other guy who had cancer of the kitten.”

Thinking about it, I generally only share really upsetting news with friends and almost never online.