I would actually argue that negative content is more popular than positive content. Hence the popularity of all kinds of doom and gloom. However, the biggest determinant is how it engages your emotions and your biases (especially your confirmation bias). Stuff that is written using logic and facts, doesn't have this. Stuff that makes you angry or engages your other emotions, even if untrue or poorly argued, is the one that people "share" or "like". That's also why guys like Trump get votes, while other more thoughtful potential candidates don't even show up in the polls. Who wants to hear actual solutions, when they can just get angry?