There's a second effect: nobody likes a whiner. This truism to the limit it applies, disfavours negative emotion content being posted by individuals because we're acculturated to it. (Disliking whiners)

My mental health professional says the rosy tinted spectacle view of the human psyche is a wonderful thing and why unlike Arthur Koestler and Primo Levi we stand a better chance of a long and happy life than a long and sad one. Arthur and Primo aside, unhappy people aren't all stoical and long lived.