People that feel entitled to you bending the rules for them.
No, Karen, I'm not going to ignore the rule that it has to have been 48 hours since vaccinations to give your dog a bath, especially since you've been a bitch ever since you walked in here. If you were a regular who tipped well we might be able to work something out, but I can tell just from our three minutes of interaction that you're the type of bitch that if you find something wrong with my work, you'll use the fact that I did you a favor to fuck me over.
Basically the more somebody wants me to go out of my way (and put my job at risk) to help them, the less inclined I am to actually do it.