At the OP, sounds like your girlfriend might have been a waitress at one point...
Many servers in the US depend on those tips (the base pay is lousy) -- but they should give good service to warrant it. For me, it isn't a given.
If I have to *ask* for a refill of my water or tea.... that's a major negative. If I have to *ask* for my bill after I've sat quietly or tried to signal.... negative. I don't blame the waiter for kitchen/chef screwups. My wife spent a fair amount of time waitressing in her teens - she is usually quite generous but can be brutal to servers who don't do their job properly.
I start at a reference of 15%... it can go up or down depending on how I'm treated. I've been known to drop 30%. I don't leave *nothing* - a waiter may simply think you forgot.
Tipping is capitalism at its purest - you're being given a service, the waiter is acting as your go-between with the restaurant (who pays them just enough to hang around).
Yeah, I prefer the Japanese system .... but when in Rome, you do as the Romans do. And not tipping is really poor etiquette in the US.