Here's a wild guess:

• For evolutionary reasons, for most people it's easier to trigger a negative emotion than a positive one. So, regardless of their situation, the average person is suffering much of the time, and is either resigned or angry about it.
• Questions like "What does it benefit anything if I am miserable about suffering?" are meaningless to the average person. Emotions are reality.
• Unlike real life middle-class Western culture, the reddit community doesn't give any particular status credit for being upbeat, so the usual motivation for not acting negative all the time is gone.