It's the attention. As much I hate Jason, he probably has the right idea - though he uses block too liberally, even with people who just politely disagree. All they are looking for is a reply, so they get more and more pathetic in their attempts to provoke one. And as soon as you snap, they get their moment in the spotlight, so double down. Twitter was neevr meant to be a place to have conversation, it was meant as a one-to-many personal publishing platform where you could follow people and just get a glimpse into what their 'status' was. Like out of interest see that a celeb has gone on holiday, or a sports star had bought a new car.
Then it became a platform for vanity where people chased likes and started to read all the comments and sycophantic fan adulation. Then you got trolls who would comment on those comments, then take pot shots at the person who posted the tweet (e.g. opposing fans), then they were left to try and drown those out with more likes and so it became a platform for preaching basic 'profound' statements that would garner those likes (things like 'racisst are bad woo!', 'sexism is bad'). Then anyone who trolled/disagreed could easily be dismissed as being in one of those groups.