Humans, by design, cultivate orthodoxy across cultural lines which usually correlates with emotional investment on the parts of individuals. Put simply, when pe

Humans, by design, cultivate orthodoxy across cultural lines which usually correlates with emotional investment on the parts of individuals. Put simply, when people can't agree to disagree, they squabble, often viciously. Downvoting facilitates the formation of islands of like-minded thinkers quickly on websites that allow people to connect to each other (any site with groups/friends). For something like Reddit, which is literally a collection of islands like this, the tool is a good fit for building cohesion. Without downvoting, people will still resort to being unpleasant to each other to create homogeny; they find like-minded thinkers, group together as friends and are unpleasant as a group to people not in the group (how you can have definite cultural groups on twitter).