The difference is that you have a much more personal relationship with the server than you have, say, with the clerk at the local supermarket or drugstore.
Having worked in a restaurant, I can tell you that it’s not easy to get the food out hot and on time, and it’s very easy to fudge a customer’s order. There’s a lot of pressure and sometimes you have to beg the chef to get your order out promptly. Depends on where you work.
If you get good service, tip well (because tips are shared and everybody, not only the server, benefits). If you don’t, don’t tip but make it clear why you didn’t. This may not be the fault of the server, but letting management know about this will also put pressure on the other people in the chain (ranging from the chef to the busboy).
No, I do not. In both cases I am a customer who simply wants to exchange money for goods and services.