I work in a doctors office and the folks I work with have the most negative attitudes. No matter how slow it is they will complain, when someone says to me, Gee you must be busy, I usually reply with, No, but there is an obligation to complain.
I find it really hard because I really try to put on a happy face for these folks, for some of them I may be the only person who takes the time to talk to them, they are sick, many are lonely and I really think that our office should be a sort of haven for them where they can share their worries.
When people thank me for being nice I just tell them that it is just how I am, and that I hope that when I call my doctors office there will be someone like me at the other end.