Tipping is governed by two separate races to the bottom:

* Buying social prestige amongst your group by showing generosity.

* Buying better service by showing generosity.

Both these result in motivation in favor of tipping more and against tipping less. Both of these require your tips to be above average, and not everyone can be above average.

What makes it somewhat tricky is that tipping to show generosity is a type of virtue signalling. And virtue signalling generally doesn't work if you admit you are doing it for self serving reasons. Since the best liars believe their own lies, our brain is resistant to acknowledging that these are the true reasons behind tipping. It prefers to pick justifications that align with the virtue being signalled.