Everytime I dismiss a roommate a different roommate is in my apartment
Solution 1:
I had the same issue about a year ago. No, you dont need a roommate if you don't want one. When your roommate option went away had you done a cheat before then? If so, it has glitched your options. While cheats are great and help more often than not, they do mess with your game. You can go to your game options, settings, restore default. This will take the options back to the original ones. You will have to set them back to your preferences after the fact, but this should bring your "turn off roommate" choice back.
Hope that helps. ☺