I run a small mastodon instance on https://dog.estate. The reason I started it was because the instances I wanted to join didn't approve me and none of the other that was open for registration at the moment felt right.
So I started my own. For a long time it was just me. These days it also includes two online friends of mine.
I have no plans of running anything large. Anyone I know isn't terrible can potenially get an account, they just have to ask me.