Stress
Keeping things clean isn't as easy as it may seem. Janitors are under a lot of pressure to keep the grounds where they're employed not only sanitary, but also aesthetically pleasing and safe. Anything less will have them in hot water. Meeting these kinds of standards means spending hours and hours tackling your never-ending list of tasks—and then not getting mad when somebody messes up all your hard work.
Whether it's a three-year-old accidentally spilling soda all over your freshly-polished floor or some snobby adult throwing their trash in the potted plants because rules are for children, your reaction has to be the same: get your cleaning supplies, deal with the mess, and don't get mad about it. Unless you're the really chill type, that kind of stuff is going to give you some gray hairs after a while.