PDT 2/2 : Why has tipping increased?

[El Diablo]

OK, so here's the deal. I was pondering two questions today. I thought there was a slight chance these threads might make for decent EDF threads, but a good chance that they quickly result in just pretty retarded discussion. But, whatev, I can lock threads, so here goes.

Don't answer this question unless you have an economic/business/sociological aspect to your post. PEOPLE TIP TOO MUCH!! or PEOPLE ARE CHEAP!!! or F TIP JARS AT SUBWAY! or whatever are all idiotic responses, so don't make them. Thanks. If you think the thread sucks, just ignore it, it'll get locked soon enough if it sucks.

Also, if you don't agree with the premise of this question, DON'T RESPOND IN THIS THREAD, thanks.

So, having said all that, here's the question.

Why have standard tipping amounts/expectations increased over time?

When I was growing up, pretty much through college, 15% was relatively standard, but it was not uncommon for people to tip 10% and it was very rare for people to tip over 15%. Friends who waited tables would be very surprised to get tips over 15%. And tipping pretty much just existed at sit-down restaurants.

Nowadays, 15% is like a baseline tip. That's pretty much expected. Especially at nicer places, 18%+ is often the norm. Friend who wait tables at places like that often expect 20%. Also, tip jars now exist at all sorts of low-service places.

So, the question is, why has this happened? Tips rise proportionately as prices increase, and eating out is definitely more expensive, so it's not to keep up w/ inflation or anything like that. But the societal norms of tip percentages have crept up.

Why is that?