Stress
If you've been through the motions a couple of times, there's not a whole lot of pressure for the touring part of the job. You have to make sure the next day's plans are good to go, so expect to stay up late—past the time when most of your tour group has crashed for the night.
You need to lay the groundwork for visits, contact venues, restaurants, and arrange transportations. Are you well organized? Great—that's only one part of the job.
Then there's the other part: the actual human guidance you have to provide. Let's face it—not all tourists are pleasant. Some will be grouchy, some will complain. They'll bring ill-behaved children who scream, don't listen, or possibly just stay on their tablets for the entire tour, ignoring every word you say. They might even bring what appears to be an incontinent dog.
Minutes after you pass the last rest stop for fifty miles—which you'll have warned them about seven different times—they'll suddenly realize they need to relieve themselves.
Oh well. This is when you get to tell them they can either use the bottle or hold it.