If I start acting like a jerk, them my users will start leaving. Pretty soon I don't have anyone left except people who like being treated like that. In my experience that will probably mean they're jerks too.

And thus pretty soon my instance is "that instance run by that jerk who lets her users be jerks". At that point, admins on other instances stop federating with my instance. Eventually the only other instances who are willing to talk to mine are also run by and full of jerks, and I am amidst a hell of my own creation.

So that is, as I see it, the external forces acting on me to try and be a chill admin who's nice to her users. It's a lot easier for them to leave my server and stay in contact with people elsewhere than it is to leave Twitter.