I don't remember where, I think the reason 25% sticks in my mind is because I usually do 20% and tend not to think about it. I did stop going to restaurants because of it but I would still go to fast food / take outs and then they started requesting tips before they even made the food. The increased in cost on top of the tip felt like small scale extortion - what are you going to do to the food if I don't pay. So now I just cook at home and save a bunch of money and I wonder how many others are doing the same.

I was always taught that 15% is the standard and that's all I give.

It was in 1995. We briefly flirted with 18%, but it's 20% now. At least for restaurants, taxis, and barbers. I've been prompted for tips at self checkout kiosks and tip a generous 0%.

Why should the percentage change over time?

Given that it's a percentage it already accounts for inflation, so I see no reason to ever change the default.

18-20% is already completely insane.

10% is the norm here.