I was thinking about how much politeness might mean to a server.
I do try to please and thank you them and be pleasant. Once in a while I will comment on something they are wearing, what is happening in the restaurant etc. just to have a little attention could be nice for them I think.
I have often thought how much an extra dollar in their tip might mean.
On the other hand mistakes are seldom their fault and I have to remember to direct my unhappiness to the manager and not them.