In my teenage years I worked as a busboy, and part of my wage was a share of the wait staff's tips.

I also worked McDonald's, including the register, and it never occurred to me that anyone would ever tip me there. (Not that at 20 cents per hamburger or regular fries a tip would amount to much.)

And this perhaps is why I have always tipped 20% for table service, and begrudge tips at the counter.