Tipping and Power
The New Yorker magazine takes a look at the "social institution" of tipping at restaurants. It looks like tipping goes beyond rewarding good service. Here's a copy/paste:
...when it comes to tipping, who we are and how we feel matter a lot, because a tip is essentially a gift, and we give better gifts to people we like than to people we don’t. Tippers aren’t trying to drive hard bargains or maximize their economic interests; they’re trying to demonstrate their status and to reciprocate what they see as good behavior.
Diners… like the power that the ability to tip gives them. Waiters like tipping because it gives them the chance to distinguish themselves from the crowd and to score an occasional windfall.
Quality food and quality service are not the only ingredients for good tips. Also, there's more to explore in the deeper feelings of the customer.