This does not make a lot of sense. 
Once you ***join*** a Stack Exchange community,
you are a member forever (well, almost forever),
and you joined Super User [ten months ago][1],
so you shouldn’t be having a problem with that. 
And, once you login to any Stack Exchange community,
you are automatically logged in to all the communities
that you have ever joined — in that browser session. 
Of course, if you log out, or switch browsers, you must login again,
but what you describe should not have happened.

Next time you’re in this situation, look at the top bar. 
If it shows your gravatar (account picture) and your stats, like this:

                ![][2]

you’re logged in, and should not be asked to log in again. 
But if you see this:

                ![][3]

then you’re not logged in.  (Be sure to select “log in”,
because “sign up” means “create a *new* account”,
and you don’t want to do that.)

P.S. Thanks for saying that we’re nice!


  [1]: https://superuser.com/users/408673/kitty?tab=profile
  [2]: https://i.sstatic.net/5y6jV.png
  [3]: https://i.sstatic.net/eLqD9.png