When I worked in the restaurant industry I remember a fellow waitress who would get extremely upset if she received a low (or no) tip from one of her clients. It would ruin her whole night, maybe even her whole week. It was so bad that you could visibly see it affected her ability to work that night.
Her problem, I think, was that she always expected an average or good tip from a customer. A better approach would be to expect nothing from each customer so you were never disappointed.
Just be happy to help. Give, without the expectation of receiving.