Some people like to have control over others, and get a kick out of enforcing rules they've made. These seem to make up the bulk of Reddit's moderators. Others

Some people like to have control over others, and get a kick out of enforcing rules they've made. These seem to make up the bulk of Reddit's moderators. Others do it more casually, just because they wanted to have a forum for a topic and you need to do some light-touch moderating to keep the spam out.