Stress

For all the furriness, fuzziness, and feathers, pet sitters can actually encounter job stress. As they sit in traffic several miles from their next appointment, they wonder if they can make it to Rover's house before he loses the battle of the bladder. Worrying about all the stains that might appear on a client's carpet is enough to set any sitter on edge.

 
We know what you're plotting. (Source)

In some cases, the pets themselves might stress a sitter out. Every time a sitter enters the house of an ill-tempered, super-sized mutt he can't stand, he can't help but think that the dog's in there waiting, plotting, scheming against him. "Can he read my mind?" "Does he know he's making my skin sweat and my heart pound?"

And yes, sitters can even experience stress in a very successful pet sitting business. 

They might panic when they look at the day's appointment schedule, wondering how they'll fit them all in and still make their doctor's appointment or grocery store run. They might wonder how they'll pay their bills when their appointment book frequently displays acres of blank lines. The best sitters can put that stress in perspective—they've taken the leap to start their own business, something many people only dream of doing.