I think it's just human nature to get enjoyment at making other people upset. It comes from a lack of empathy, understanding, and perspective.

When I was young, my cousin and I would hop into randomly chosen public chatrooms on MSN, or Yahoo, and just start typing stupid messages. We'd spam the chat with constans messages of "booger" or "poopy fart" and watch people get annoyed with us. Sometimes we'd pick a random message from someone and call them out telling them "hey {username}, shut up stupid." The whole chatroom would get mad and tell us to leave, or to stop, and that made us keep doing it more. For a good half hour to 45 min, the entire chatroom was having a bad time except us, who were laughing out heads off at how mad they got and how compeley powerless they were to stop us.

We were also 10.

We haven't experienced how annoying and frustrating that actually is. We didn't understand or even care just how disruptive we were being, nor did we care about our contribution to making the space a bad space to be in. We, as children, didn't have the empathy, compassion, or perspective of experience to care about that, and were just reveling in the attention and the power to force a group of strangers to focus on us and not what they originally wanted to.

Some people eventually develop empathy, self awareness, gain perspective on the world, or otherwise come to understand how immature these acts areof getting joy at being annoying, and stop. Other people don't. The internet is home to people in all different stages of their life's journey, and a lot of them haven't reached that point yet.

Some troll because they're immature. Some do it because they actively dislike a community and pettily get joy at annoying them. Some people just like the attention. People are complicated and weird, and often hard to understand. There is just one thing that will always be true:

As long as people exists, so too will trolls.