When you leave a tip, you are acknowledging the contribution of another.
It's a big aspect of motivation.
We'll continue to do what we are motivated to do, but that motivation will wane when our efforts are not appreciated.
The biggest tip I ever got was $1000 from a client. This was several years ago, but to this day, whenever he needs something, I always respond and try to put his request at the front of the line.
The truth is that money itself isn't the motivator for a lot of us - but rather, feeling appreciated.
My wife and I have a mutual appreciation for one another. I think this is one of the keys to our long marriage.
We also have a shared appreciation for live music. As such, we make a point of always putting a tip in the jar for a performer we appreciate. We want to support them however we can.