Anger compels people to complain and be loud about it. It's more visible / noticeable. Anger and hate also follow with requesting changes. If you want something to change you need to speak up to get it done. So when you're angry about something that needs to be fixed you have a lot of good reasons to communicate that.
Having fun is less compelling to communicate. It can also be seen as bragging that you're having a good time. There's less incentive to go out and tell everybody about it becuase you don't need to change anything.
Anger is viral, joy is not. 10 angry people will be louder than 100 happy people.