Serving the public can often be a thankless task. If you don't think so, spend a week as a waiter, a cashier or any kind of customer service rep.
That being said, and human nature being what it is... I would bet that even if a waiter or waitress provided EXCELLENT service all the time, he or she would average less than 15%. Of course, no one on this board tips under 20%

but (gasp) there are people that do.
With that in mind, I like to approach each dining experience with a positive attitude. First and foremost, our family is enjoying a meal together. That in and of itself is the most important aspect of the event. Secondly, we're either returning to a favorite restaurant or trying some place new. We're either expecting the same great food we know, or hoping to to try new food that may become a favorite. Lastly, we have someone serving us who may or may not be swamped, and who may or may not be new, and who may or may not be attentive. As diners, we have no control over the first two aspects of the waiter or waitress' circumstances. We do, however, have control over his or her attentiveness. If my kids are on the thirsty side, dadgumit, I will respectfully make sure my waiter or waitress knows this, and allow him or her to alter how he or she serves a table to suit us. It's really that simple.
I don't really care if 18% is added to my bill. It saves me the trouble of figuring it myself. I know how hard it is to please the public. I'm glad I'm not the one waitressing. Overtippers and perfect children are not the norm, despite DIS statistics heavily skewed the other direction.
YMMV